I was mountain biking today got tired and decided to set down for a few minutes when I got back on the rear tire wouldn't spin. Turns out the chain was jammed between first gear and that plastic ring that is between the cassette and the center of the wheel. Luckily I was able to pry the chain loose and was able to go on my way. Question is what caused this to happen and can it happen again?

When the rear der allows the chain to move past the large cog it get's wedged between the cog and the guard/spokes. If the der isn't well adjusted or has been bent inward then the large cog limit screw is likely not set correct to limit the chain's travels. The other common way the chain derails when the bike is set down is that gravity tugs on the chain's length and combined with a back pedal can lead to "backwards" shifting. Picking up the rear wheel off the ground and pedaling a stroke or three usually get's the chain back on the cog it was last shifted onto. Andy

Two possibilities. The derailleur may have taken a hit and is now out of alignment, or the low limit screw is turned out too far allowing the derailleur to overshift the large cog.

You laid the bicycle down on its right side while you rested, pushing the derailleur and chain past first gear. The derailleur mount may or may not be bent.

If its not done it when you are changing gear riding then as above if you laid the bike on its left side, with the chain on the small front chain ring, it only needs a touch on the RD the chain can then loose tension & just falls into towards the spokes.

As far as I can tell this might be a one time thing. The rest of the ride was uneventful.