It has been mentioned in several posts/comments by Draginol that the units assaulting the colony will sometimes panic and stop or reverse direction if a lot of them are being destroyed.  Some posts have been made reporting this behavior as a bug.

I did some testing on Mission I to check out different graphics settings and just put down a few chain guns.  I noticed a lot of panic in one of the assault waves, so I tried it several times just to watch it.  I then recorded some to get video I could review.  I saw some interesting things, including the chain guns having trouble deciding what to shoot.

I have edited three of the videos to show just the panic.  Here are the links to them.

Panic 1
Panic 2
Panic 3

I didn't notice this until I was watching the videos, but Panic 2 shows panicked units reversing direction and going all the way back to the first chain gun.  It doesn't show well in the video, but it is there.

When you have finished looking at the panic, check out the assault wave information bar at the top the screen and see how the game handles the location of the wave that is panicking.  It is really weird in Panic 2.