Ranged-attack initiative and sequencing for monsters and NPCs

Question

How interesting that after over 20 years of playing RPGs, new basic rule questions still crop up...:

The PC group I'm DM-ing got into a fight with two bugbears and eight goblins in a long 10-foot wide tunnel with corners, etc, where it's more advantageous for the gobos to shoot their short bows than go melee straight away.

I roll collective initiative for the monsters which is incidentally super high, so monsters come first.

Their starting position is all bunched up in adjacent squares. So on their initiative, I move the monsters one by one closer to their PC target. Each one, after his movement, fires their short-bow.

Here is where it gets tricky: Moving the monsters individually on their collective Initiative creates straight lines-of-sight to the first PC target, enabling more than just the two goblins who stand in the front row to shoot at the PC without incurring a penalty for cover (for being in the way of each other's lines-of-sight).

The players protest, saying that given the monsters all move on the same initiative, I should assume they move at the same time and maintain their initial formation, and therefore I should assume the goblins shoot before they move, giving all goblins, except the two standing in the front, partial cover penalties. Alternatively, I should roll initiative for each monster individually...

I do admit that I probably should have separated the two bugbears from the eight goblins, in this case, however, it would have made no difference in terms of the numbers being able to get a clear shot.

I hope I'm making myself clear? Am I not playing RAW? Am I unintentionally making a house rule here?

Answer 1

Your ruling was correct

Although there is no such thing as "collective initiative" in 5e. Instead, multiple creatures in one group can have the save initiative score. If initiative scores are the same, DM or players decides who acts first:

If a tie occurs, the DM decides the order among tied DM-controlled creatures, and the players decide the order among their tied characters. (PHB. "The Order of Combat")

At first glimpse, this contradicts to the "acts at the same time" phrase from the same chapter:

The DM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time.

However, simultaneous actions was the 2e thing. For the sake of simplicity, 5e doesn't use this concept any more (it has no rules for that, also there are no secret rules). Despite combatants acting simultaneously, the DM is supposed to resolve combat sequentially:

During a round, each participant in a battle takes a turn. The order of turns is determined at the beginning of a combat encounter, when everyone rolls initiative.

4.Take turns. Each participant in the battle takes a turn in initiative order

Answer 2

You've already realized one mistake you made: the bugbears and the goblins are not identical creatures, like the initiative section specifies:

Initiative determines the order of turns during combat. When combat starts, every participant makes a Dexterity check to determine their place in the initiative order. The DM makes one roll for an entire group of identical creatures, so each member of the group acts at the same time.

How do they act? You handled that part correctly. While you're rolling as a group to speed play and simplify turn order, the creatures are all individuals and all have individual turns, just with their initiative tied by default. There's a later paragraph that covers how to break ties:

If a tie occurs, the DM decides the order among tied DM-controlled creatures, and the players decide the order among their tied characters. The DM can decide the order if the tie is between a monster and a player character. Optionally, the DM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first.

The thing to remember is this: other than reactions, one character cannot do anything during another characters turn. While the story says that all the craziness of combat of a single round is happening in the same six seconds, for play purposes it has to be divided up - one character goes, then the next, and so on. Nobody goes at the same time as anybody else.

Speaking of dividing things up, I suggest smaller groups. I generally try to keep the groups evenly sized within type, but also roughly the same size as the party. For instance, if your party was four or five characters, I would have made the goblins two groups of four, and the bugbears as a third group. If you had a bigger or smaller party, adjust accordingly. The idea of more, smaller groups is to (hopefully) spread the character actions in among the clustered NPC actions. That gives the players a better feeling of control of their characters' fates.

_{As an aside, when I DM via Fantasy Grounds (a VTT application), I use individual initiative for PCs and NPCS alike. FG handles initiative rolling and order tracking automatically, so you get plenty of interleaved turns, but it's too much work in face-to-face play.}