I guess "spadł" comes from "spadać", but in all web pages spadał is shown... Also spadało instead of spadło... What's going on!?
"spadł" comes from "spaść". It's perfective. "spadał" is from "spadać" and is imperfective, thus referring to the 'process of falling down'.
Can I say "wyleciał"?
"wyleciał" could be used if it fell 'out of somewhere', e.g. you were carrying a box full of things and a fork 'fell out of it'.