It's a scripted sequence. It's the developers taking an opportunity to remind you that you will be tackling more than one of these assholes from this point onwards. Time constraints, budgetary reasons, technical issues - you name it - there could be any reason why they chose to do so this way.
Personally, they could have gotten away with a scripted sequence by showing you 2 Aliens in a cut scene or have them both be on the other side of a pane of reinforced glass and you could watch them trash a room of survivors...
It's one of the weak points in the game - having the 2 Aliens drop in on you while navigating the wrecked tunnel and have them not react to your interactions didn't sell the experience very well to the player. One of the few weak points, that is, that break the game a little. There were better ways to achieve the same thing but this was the chosen method.
Not the best but, hey, that's game development for you.