As far as the USMLE goes, there are a few reasons why performance would be down:
1) students arent as smart (i dont believe this)
2) training is subpar (this is possibly more likely)
3) learning disciplines are not geared towarde USMLE (more likely)
4) extra stresses/material (very likely).
In my mind, the USMLE was harder than the COMEX, but certainly doable. However, it was difficult taking it so close to the COMLX, and trying to flush OMT and store up on biochem. Obviously, DO education can be fairly weak on the research-based subjects (cel bio, biochem, genetics), but these barely account for 5-8% of the exam. I think the real trouble comes in preparing for, and sitting for, two completely different exams testing over the same information, for the most part. Very difficult to hop from one to the other without at least a few weeks in between for a change in mindset.