This question comes up sometimes in many RV forums. I have heard everything from "moisture in the genset", to "cat stepped on the switch on the unit", to "auto start" (that some units have) to "radio waves causing a spontaneous start".
We had a 2000 Itasca Horizon with an Onan 7500. Walked out of the house one morning and heard a humming. Now the sun was shining, not a cloud in the sky and we had just returned a day before from a trip, so the batteries were up. We have no cats or any pets. The Onan was running like a champ. I have no idea why, but I do know it happens.