Originally Posted by Crack'n'fail

sitting on the nose of the seat is where you get the most leverage on your pedals, it's where they slide to when they are digging deep either due to fatigue or desire to put in an extreme effort.

I move back and forth between sitting and standing on climbs a lot too. I'm about to turn 35 and I think that as you age the back gets a little less forgiving and you need to shake it up more. Lance is getting up their in pro-cycling years, maybe that's part of it?