PHB p. 195 states that involuntary movement does not provoke opportunity attacks:
You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.
Is there a general rule somewhere about involuntary movement and how it interacts with other effects?
The question arose at our table about what happens if a creature is dragged via the Thorn Whip spell across an area of Spike Growth. Does the involuntarily moving creature take the damage from the spikes as if he'd voluntarily moved through them?
Logically, yes, if you're being dragged through spikes, you take damage from them as if you'd walked through them. But logically, if I drag you past my ally, my ally would be able to make an Opportunity Attack, likely more easily than if you were moving under your own power. Presumably that is not allowed for balance reasons rather than versimilitude reasons, so I wonder if the same should apply to other effects?