Well, since the general public was on the huge Silva hype train you could get a good odds betting Weidman. This since Weidman was relatively unknown for the broad masses. Also take into account that many of the top UFC fighters (GSP, Gustafsson etc) predicted Weidman winning.
So probably someone with a lot of money started believing what the fighters said. Probably the guy who made that bet had seen Weidman on training sessions as well.
I say, congrats on a great and huge bet win!