Originally Posted by FBinNY

This could be classic chain suck, where friction keeps the chain from disengaging at the bottom of the chainrings and it gets carried up and jammed into, or past the chainstay where there's limited clearance.

It could also be that you somehow dropped the chain and got it jammed in there.

Wet conditions stream crossings and loose mud are notorious for causing suck, because water washes out lube, and carries fine silt into the chain. When the water evaporates it leaves the silt behind which increases the chain's internal friction and causes suck.

There could also be other causes, so this is simply a general analysis. Better or more specific analysis, would require seeing the bike.