My interpretation of the book is that the Defense Professor looks just like Quirrell. If this is the case, then maybe it takes more and more out of him to maintain the illusion that he is someone else. Or maybe he actually inhabits the body Quirrell, and Quirrell is slowly fighting back.

Then again, I still have a hard time reading the DP as actually being Voldemort, so take my instincts with a grain of salt.