first of all that has never happened and second if you tightened it up correctly the it should'nt slip and even if it did it would put the slightest bit of pressure on the top of the collar, not the welding on the tt/ss
But this crack would not be caused by the seat post being pushed into the frame, it would crack where the frame has been cut for the seat clamp, so in this case it has nothing to do with the seat being slammed, my suburban frame cracked in the same place with the seat raised up.