In the past, I have suffered from mild dysphoria after long rides. I have to say that this doesn't include centuries though. First time I noticed it was after a ride of over 250 miles, and then I noticed it after my first 600k brevet. It wouldn't surprise me that it happens to people after centuries, and I don't know what causes it. I'm glad to say that it hasn't happened to me recently. I have heard of this happening to other randos after long rides.