I am by no means any sort of expert on thermostats or incubators, but I agree- I don't see how or why 'humidity' would influence your t-stat, but what about the length of time you had the incubator open, making adjustments (water bottles?) or observations? If I'm not mistaken, some kinds of t-stats might "react" aggressively (rather than incrementally?) to re-establish the temperature it was set for?