Please take a look at this thread. The OP placed a bounty on it that expired yesterday. I was late to the question (as usual) but posted a good answer. The OP thought so too and accepted the answer. But was probably not aware of the new rules and never assigned the bounty. I tried to warn him to no avail.
This morning, I found the bounty auto-assigned. Marc got it but it is completely unclear to me why his post was selected. It was neither the accepted answer nor the highest up-voted answer nor the first answer.
It would have been a bit tragic if it would have been awarded to the highest voted answer in this case, but that's another matter. Please consider that iff the OP was the originator of the bonus and accepted an answer then to select that answer to auto-assign the bounty to.