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?