Premature resignation?

I recently completed a game at Chess.com and was surprised when my opponent resigned unexpectedly at move 20.  I was about to force a win of some material, but I thought there was a lot more play left in the game.  What do you think? 

I've tried to annotate the game with my thoughts (during the game) and some additional comments with the benefit of hindsight (see the varations analysed in the 'MOVE LIST').  Comments are very welcome - thanks.

 Just for fun, I Simpsonized myself.

Or another possible continuation after move 20 could be:

This also wins a piece. I think white just realized that he had made a decisive blunder and decided not to hope for the same blunder from his opponent and resigned. 

Thanks Eugen.  It's often difficult to know when to resign and perhaps it was the right thing to do in this case, but the end seemed quite sudden to me.   I would have been tempted to play on a bit to make my opponent 'prove' that he could see the same continuation that I could!