It's really close on the front one to almost falling out all the time, but if everything is ok on the truck, it won't.
If yours is popping out at all, you either have wear issues or your chassis is tweaked.
There are a few wear items that cause the clearance to be enough to where it will fall out. The little diff hanger can wear/crack, which allows the diff to flop around. The cup itself on the trans wears, the ears on the dog-bone wear, the CV joint wears. Any of those can cause a sloppy fit which will be prone to falling out.
That in mind, I've run probably about 8+ gallons through my aftershock (similar core to the LST1, LST2...) and the only time my front center fell out of the cup was when I broke the chassis rails. My diffs, diff hangers, center cvd's, trans cups are all original. So it takes quite a bit of time for wear to cause a problem.
I'd suspect more that you have cracked chassis rails, cracked bulkheads, busted diff hanger or just stripped out screw holes holding things together.
I broke my chassis rails 3-4 times since I got it over a year ago. Very frustrating to replace... but is really the only solution to a broken part.
On a side note, I've never had the rear center pop out yet.