Why Do Onions Sprout Even When They’re Stored in a Cool Pantry?

I keep onions in a cupboard that stays fairly cool, but some still start growing green shoots before I get around to using them. Other onions from the same bag remain firm for weeks.

Does sprouting depend more on humidity, airflow, or how mature the onions were when harvested? I’m trying to figure out whether my storage setup is encouraging onion sprouting or whether variation between individual onions is normal.