Looks like that’s the ticket. Good call, I honestly thought soil temperature played a larger role in leaf out but looks like it’s one minor variable that is easily be overcome by the more important temperature and sun.
I would’ve guessed sun and soil temperature over air temperature.
It seems it involves “a combination of signals including the weather becoming warmer, the days getting longer/increase in the amount of sunlight, the soil becoming warmer, and nutrients & water becoming more available.”
But the most important are:
“At the simplest level, most experts agree that a combination of temperature and photoperiodic cues is responsible for the timing of budburst in most temperate woody plants.”