I had the EXACT same problem almost from day one on My 2008 Wrangler "X". I took it to the shop and they couldnt get it to do it, but they documented the problem.
A year or two later it started doing it a few times a day. It would pop out of first gear so violently it would feel like I was rear-ended. I always worried it would happen in an intersection and I would get hit.
Anyway, I took it to the dealer, who was able to replicate it. They had to replace the entire transmission. The way it was explained to me (and I know nothing about mechanics of a car), is that they teeth of the gears werent lined up properly, so it would spit the stick out sometime.
They replaced it for free, I think because it was documented the first time. If it's warranty, take it in ASAP. If it's not, I would make a case that this is a known problem and should be fixed.
Personally, I read so many people having the problem that I think they should consider a recall. It;s really dangerous, and eventually will kill someone trying to speed through an intersection.