Does the movement from Infestation count as willing?

Question

I know that infestation movement does not provoke opportunity attacks. However, I have a different question:

Does the movement caused by the infestation spell trigger the extra damage of booming blade?

The description of booming blade (SCAG p. 142) states:

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails. On a hit, the target suffers the attack’s normal effects, and it becomes sheathed in booming energy until the start of your next turn. If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.

And infestation (XGtE p. 158) says:

You cause a cloud of mites, fleas, and other parasites to appear momentarily on one creature you can see within range. The target must succeed on a Constitution saving throw, or it takes 1d6 poison damage and moves 5 feet in a random direction if it can move and its speed is at least 5 feet. Roll a d4 for the direction: 1, north; 2, south; 3, east; or 4, west. This movement doesn’t provoke opportunity attacks, and if the direction rolled is blocked, the target doesn’t move.

Answer 1

No. Movement forced by a spell is not willing movement.

"Willing movement" (and being "willing" in general) is never defined in the rules, though it's referenced in spells like booming blade (SCAG, p. 142). Thus, since it's not a defined game term, we default to the natural English definition of the term.

According to Dictionary.com, "willing" means "disposed or consenting; inclined". However, the movement from the infestation spell (XGtE, p. 158) is forced by the spell on a failed Con save; the target has no choice about whether to do it, so it's certainly not doing so of its own volition. As such, it seems clear that movement forced by such a spell is not willing.

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Jeremy Crawford supports this interpretation in an unofficial April 2016 tweet in which he addresses a similar question about the dissonant whispers spell and its interaction with booming blade:

Would you take damage from the second part of Booming Blade if you fail a save against Dissonant Whispers and move away?

Booming blade hurts you if you move away willingly. Dissonant whispers forces you to move—doesn't trigger BL.

but if they move after whispers in their turn (to get back into melee, for example) that counts, right?

Yes.

Crawford provides the same explanation in an earlier unofficial tweet in February 2016:

Would the movement caused by a failed save on Dissonant Whispers be willing or unwilling movement?

If a spell forces you to move, as dissonant whispers does, you're not moving of your own volition.

Does that mean the movement from Dissonant Whispers doesn't provoke opportunity attacks?

The movement in dissonant whispers can provoke opportunity attacks, since it uses your reaction (PH, 195).

The same logic would follow here regarding infestation and booming blade; the spell is forcing the targeted creature to move, so the creature is not doing so willingly. Thus, booming blade's extra damage would not trigger.

(Sidenote: infestation does not trigger opportunity attacks either, because it moves the creature without using its action, reaction, or movement.)

Answer 2

No

As you said, booming blade requires willing movement:

If the target willingly moves before then...

"Willingly" means:

readily; of one's own free will.

However, this movement is a result of infestation forcing the creature to move regardless if they fail their save.

The target must succeed on a Constitution saving throw, or it takes 1d6 poison damage and moves 5 feet in a random direction if it can move and its speed is at least 5 feet.

Here's the thing: the creature has no option to decide if (or even where) they want to move. Therefore it is absolutely not "of one's own free will"; rather it is a textbook example of being forced to do something, which is quite the opposite.

Rules as Intended support

Jeremy Crawford supports this view:

Booming blade hurts you if you move away willingly. Dissonant whispers forces you to move—doesn't trigger BL.

Dissonant whispers is a spell that, like booming blade, forces movement on a failed save. Thus it is a very applicable comparison and the ruling is transferable.

He also somewhat more indirectly supports this further when he says:

A push is an effective way to force a creature to enter an area of effect, unless it requires willing movement.

A push/shove is another way to make an opponent move similar to this spell. Once again the creature has no choice in the matter and, as a result, Jeremy Crawford says that the movement is not willing. I think the situation is very analogous to the one in your question.

Answer 3

No.

As you quoted,

If the target willingly moves

Infestation forces a saving throw, imposing damage and movement.

The target must succeed on a Constitution saving throw, or it takes 1d6 poison damage and moves 5 feet in a random direction if it can move and its speed is at least 5 feet.

This is pretty clear indication that Infestation's movement is forced, not willed, and therefore does not trigger Booming Blade.

Answer 4

I'm going to go against the grain here and say it would. Though perhaps it's a bit of an edge case.

The description and mechanics of the spell makes it seem as though the movement isn't controlled at all by the player. It's a response to swatting away the insects, moving around to get them off them. The random direction called out in the spell is the key here: it's not something you are controlling, and I'd personally rule that they are wilfully getting out of the area that has a bunch of insects around, just like they might attempt to get out of a stinking cloud or any affect.

Answer 5

Yes I would say it’s up to the dm. If you put yourself in the shoes of the target. They are getting swarmed by insects. It is a natural reaction and I would say an extension of ones will to move away from it. The randomness of the movement exemplifies the panicked state of victim as it willingly moves. This rule is necessary because no player is going to say “I panic and run to a place I wouldn’t normally” Dissonant whispers is another case where’s someone’s will is being imposed on them. I believe the unwilling motion to be an oversimplification for being pushed or thrown, and people are thinking too much about this since its in a pretty complicated, and not always properly explained, game.