Priceline上定的米兰到上海的机票，为什么会被取

我在Priceline上定的米兰到上海的机票，信用卡预授权成功，也收到了确认邮件，为什么过了几个小时会收到邮件说被取消了，而且我想重新再定就提示Sorry -
Unable to book your airline tickets

Airline availability and
pricing is constantly changing and we were unable to book the flights you
requested.
Please select an alternative option from the itineraries below.
Prices are per ticket including all taxes and fees.哪位大侠知道这是什么情况？