Ok, I used to be a fan. Now I'm fed up with it being too random.
I just had an awesome round going, and had 11 air attacks in under 5min, with HUGE waves coming in too. 5 Spec Ops with no less than 30rifle couldn't kill them all.
I had 3 bunkers (left right and center) at very bottom. I placed a trench directly in front of center(it had a tree touching it's left). When the engineer left range of the trench, it disappeared.