Some varieties of single trigger will double when the barrel selector is in between settings. My experience has been when the shooter fiddles with the selector between shots, and inadvertently leaves it in the middle when the gun was re-cocked.
So, My guess is a combination of dirt, parts clearances, cocking technique, and selector fiddling. Hence the difficulty of reproducing the hang fire.
Strip and clean, de-burr and polish, and go shooting.

Out there doing it best I can.