[tubafreak]

That's probably an effect of friction, or the lack thereof. The tights stick to the saddle a little bit, they slide against your shorts/bibs, the shorts/bibs stick to your butt. With out the tights your shorts/bibs stick to the saddle a bit and the sliding hapens between the butt and shorts/bibs. This is why hikers (a group I once belonged to, way back when) wear liner socks, no matter how hot, it keeps the blisters away.