2 things (that I understand) that must be present. #1-Water temp, #2-length of Day. If time of sun is right but water is too cool, it's delayed. If Temp is right but days are short-neeah--still seems to be a delay.
#3-fish know something we don't and it changes every year-fish behavior seems to change as fish grow and like us, not all think alike.
I know I said 2 things, but there's probably 50 things we may never fully have the answer to..
I think you guys know were I'm at.. haven't had time to go look yet, and 77 degs yesterday went to snow and 25 today. Probably single digits tonight but I will look tomorrow at the tumbleweeds.