I see this happening too. I always recheck the focus for each print. I know that in my case the negative isn't popping. It just doesn't get anywhere near hot enough with the dichro head on one enlarger. The heat absorbing glass on the my other, condenser enlarger does a good job of preventing the negative stage from getting too warm too. What does happen is that the enlarger head will creep down the column just a little bit if I forget to lock it in place.