Why did White resign?

http://www.chessgames.com/perl/chessgame?gid=1419414

White's down a piece.

Thank you  That makes it easier for others to see

White was down a knight and a pawn by that point. Maybe didn't feel like black had too big od a material advantage?

Black has been a piece up since 10. ..Qxd4.

Edit
When I started typing nobody had written it down yet. 

Hi mmmjv--it's not obvious why white chose this particular turn to resign, but he's been a piece down with minimal compensation since his blunder nine moves ago.  18...Nc6 left the whole center guarded and the c8 bishop free to come into play; against a strong player this is inevitably lost.

White missed that after 11. Bb5+ Nfd7!, the black queen is defended.

He is but he lost that piece a while ago

So it is that blunder he made several moves ago then. Thanks everybody 

No, as ViktorHNielsen pointed out in post #8 Nfd7 would save the queen.