I think it's because the parts get hot, expand, and you just don't get that disk free enough, so when you hit the gear, it's trying to move the car.
Me thinks a slight firewall adjustment is required ( while it's hot ) and it better be the right adjustment, or the car will drive funny cold. I just went through this last fall and didn't adjust it right either time and had to have Sutton reset my clutch as well as promise not to touch it again.
:alcoholic :alcoholic