That's all that a priori knowledge is: stuff you know without knowing why. Or to make the levels of abstraction more explicit, a priori beliefs are beliefs you have without knowing why. Once you start thinking about them, asking why you believe something and finding reasons to accept or reject it, it's no longer a priori.

This is a nice concise statement of the idea that didn't easily get across through the posts A Priori and How to Convince Me That 2 + 2 = 3.