Why did black resign?

In the game below, black resigned after the move Bb4.

I have another game against them, and in that they said they made a big positional mistake.  I can perhaps see some prospects in attacking the square in front of the black King, but nothing clear cut.  Any thoughts?  Am I missing some blindingly obvious tactic?

Cheers.

black is even better for me.... he has a central pawn

White can force a knight trade for the bishop, but white would not necessarily be better. I think the player of black overestimated the power of having a bishop as opposed to a knight.