It will do none of us any good to come to wrong conclusions as to WHY this happened.
WHY did the voters of this district choose the RAT?
The voters didn't choose the pick. GOP party insiders did the picking. That's why Hoffman ran on a different ticket.
Dede has a LOT of personal friends and allies who saw the whole Hoffman thing as a betrayal and voted for Owens or sat it out. They were enough to swing it.
Freepers may not like this sort of friendship-over-ideas motivation, but not liking it does not make it go away.
I am willing to bet that a significant number had already voted absentee before Dede got out of the race. I think the voters in NY will be just as surprised by this.