If no more "fixed mileage" rewards are available, then AP will default to Priority Rewards for the date in question.

I see this happen quite frequently for last minute YUL-YTZ flights where only priority rewards seem to be available a couple of days out.

The 10 priority rewards things is a real pain - especially since they can be eaten up in less than a month on such a minor route such as YUL-YTZ.