There isn't much more to say...I've found it happening with both PPro users and FCP users. In trying to capture (all at once) video and audio that runs long, the sync starts to drift about 25 minutes in when viewing the captured clip.
I realize that capturing in shorter lengths is the way to work, but is there a hardware or software reason why the sync would drift?
Depending on how your NLE software handles audio, in some cases minor differences between the reported and actual audio sampling frequency has been known to cause sync issues with very long clips. For example, Canon XL1 (if memory serves correctly) differed slightly from 48k and caused timeline grief as a result with older versions of Premiere when captured using certain proprietary systems.