My seatbelt used to do the same thing.
I was working on another project and had to unbolt the reel from the floor of the Jeep.
The reel wound up as far as it would so it was hanging about were the door striker is.
When I pulled the reel back down to re-bolt it to the floor, the belt retracted all the way for the first time!
I'm guessing it somehow retensioned itself by being forced to take a lot more seatbelt than it usually does.