LOL - First off, I am
NO expert... I am just a guy who has worked on more than a few and knows some of the inner workings. For an expert, talk to Dave Lauck of Masood Ayoob....
The spring that popped out is your Buffer Spring. There is a Recoil Buffer Detent Pin & Spring that goes into the Lower receiver. ( See Pic )
Your stock threads in to cover just the bottom lip of the Detent, keeping it from springing out, but the Pin portion sticks up to hold the Recoil Buffer and the Recoil Buffer Spring in place.
What I assumed happened is that the Recoil Buffer, which has multiple flat sides, caught a flat side on the top of the Detent, and came out. If you slide it back in, and make sure a round side of the Buffer is on the Detent, it should stay all day long.
If it does not, I would recommend checking your stock and making sure it isn't threaded too far in, making the Detent incapable of doing it's job. If that is sound, I would recommend getting a new Pin and Spring. It's probably only the Spring that isn't strong enough, or is worn out, but they come in a kit for like a buck and a half...
Give that a try and see if that fixes the problem...
JD
PS - If it's just a flat spot that caught and that is why it came out, this is not a problem. If you need a new spring, it's less than $2 plus shipping to fix. This is not a big deal, you definitely don't have a defective AR or anything to seriously worry about..
