Do you think it might have been the amount of steam they were generating from throwing too much water on the rocks?
Well... if you keep your head almost above the stones while steam goes off... Steam travels to ceiling first where it starts to fall back down, and thats the time when you usually feel the increased heat.
Maybe, I can't say. The size of the sauna, amough of water thrown to stones, actual heat (thermometer might have been broken).