I don't think that's a glitrch, I think it's the way they designed the AI this time around. That happens in many places, though most aren't as obvious because the point they won't go past is usually a doorway or large opening to a new area.
The only reason I can think of they would script them that way is so you can't sit back and pick them off little by little. They wanted a swarm effect in this one where such things increase shock value. IMO it only makes the monsters look scared or timid at times though.