In analysis, this move takes white from +15.6 to -3.95. I see now that Qc7+ is much better, but I can't see what is quite so bad about Qxb5+ to turn a winning position into a losing one. It doesn't seem to lead to any immediate loss of material or leave white susceptible to any kind of trap as far as I can see...
Why is Qxb5+ such a bad move here?
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Look at the difference between Qxb5+ and Qc7+.
Qc7+ Kb4 (forced) ; Bd2+ Kb3 (say) ; Rh3+ Kxb2 ; Qc1# That means Bd2+ QxB ; KxQ
Qxb5+ Kd6 (forced) ; Qd7+ Ke5 ! But now what? Black is now threatening.
You see the lack of black king safety is worth the piece, once the king is safe, the eval then sees black as a piece up and the white king is now exposed.
Qb5 lets black run to kd6, away from most of white's remaining pieces. With only the queen and pawn, white can't even get a draw by repetition. Qc7 forces the king to walk to where the rest of white's pieces are at. Kb3 just looks like a spot to die in. You may not see rh3 right away, but you should be able to tell its a really terrible place for a king to be.