It's my understanding that in some instances Priceline has to send a FAX to the hotel with your booking info. This might take a few minutes and is probably done before Priceline will notify you of acceptance. And so, to conceal those cases where a FAX might need to be sent (and perhaps even received back) Priceline may introduce an equivalent delay in occasional rejections. But it was a few years ago when I read this, so things may have changed by now.