This is a standard problem in AI behaviour. They are prety much hard coded to advance towards the enemy. When you think about it, this is more or less essential to make the game fun to play. BIS are good enough of course to make it much more complex than that, but that's basically what happens.
Making AI do what you want them to do here is difficult, though it has come up before and a really good effort with the forum search function might bring up something useful.
Try setting them to behaviour "combat" when the shooting starts, which will encourage them to stay prone. You could also run a looping script, something like this:-
#loop
"_x doMove getPos _x" forEach units loonGroup
~2
goto "loop"
syntax definitely not guaranteed, you'll probably have to play with it a bit but hopefully you can see what I mean. You could use the marker positions rather than the loon's own position, not sure which will look more realistic - you'll have to experiment.