that is a loose castle nut, the tab on the egg plate is what is doing that, your stock was a little loose and it was able to wiggle bag and fourth until the tab dug in and stopped it.
when you re install it make sure it is straight up and down, torque the castle nut down and stake it in place with a center punch at the little cut outs up against the egg plate.
have a look here
http://www.kiesfirearms.com/AR_Lowers.html
third picture down will show you what and where staking is and should look like.
