It might well be a AI pathing interlock happening. I've noticed this mosly in vanilla UFO supply ships with the elevator that leads from the bottom deck to the 'command' deck. If 2 or more aliens are trying to move through this section, some going up and others going down, and they've reached a point where they can't move around eachother, they'll simply won't move. Had a case where I was going in a Snakemen supply ship with 8+ blips showing up on the motion scanner suddently dropping to 2-3 each turn, nothing else was moving, once I got a visual at the lower part of the lift, sure enough there was a snakman just standing there, luckily facing away, and after killing it, the rest of the aliens came flushing out.
How this applies to this issue will depend on how many hostiles are in that room, to where they're trying to move, and how much available space there is for them to walk around eachother.