If there was only one bid on the auction and that bidder retracted their bid or the seller cancelled it then the BIN would return. I found about this in the eBay bidding board!
Recently I wanted to BIN an auction but didn't have the cash in my checking account to do so at that point so I put in a min bid to kill the BIN. Then a check I was waiting on cleared so I could afford to BIN the widget but the BIn had gone. There were no other bidders but I KNEW there would be others bidding on this widget at the end because it was very desirable. It could easily have gone way up over the BIN price. I e-mailed the seller & she agreed to cancel my bid. As soon as she did so the BIN returned and I BINned it.
Something like this may have happened in your auction. A lot of people would say its unfair to ask the seller to cancel you bid or retract so the BIN returns - like reserving the item for yourself. But that's how the rules work and they are there to be manipulated.
You may care to remember this tactic if you have more than one eBay id and you are waiting on say your pay to get into the bank. Use an ID you are not bothered about to kill the BIN, then later (when you have the dosh) retract and use another ID to BIN. Asking the seller to cancel is risky because there can be a gap between them cancelling & the BIN returning and you finding out about this. In the gap another bidder could sneak in and BIN it. Just a thought.