The U.S. dropped a lot of bombs with delayed fuses, which had the effect of discouraging attempts to disarm or remove duds. You couldn’t tell if a bomb was a dud or had a delayed fuse.
I suspect this was a dud-dud (not a delayed fuse-dud) that got buried in the mud, maybe in a raid aimed a cratering the runway.
I’ve read that up to 25% of all the bombs were duds. Just dug themselves into the mud.
A couple of years ago I was going by bike to pick up my son. Saw a lot of police, including the riot police. I stopped and they explained that I needed to cross the street to continue my journey. Being an American I, of course, asked Why?
After they explained that a BIG American bomb had been dug up I decided to cross the street and take another way home.