Thanks! Yes, it's a threaded spindle. My guess is that a little hammering during use, and some dried out lubricant (noticed during cleaning after the chuck was removed) had it tightly stuck.I don't know your lathe, but if you have a threaded spindle which I think it sounds like.
keep your threads clean.
Tighten your chuck as much as you can. Why: a chuck that spins on , on it's own hammers itself on, so putting it on tight prevents it from hammering itself. If you are slightly loose, you run the risk.