It probably was the mud that blew it up but 870s did have a problem of barrels letting go. I had one of them.
I was pheasant hunting in the early morning and basically just had assembled the shot gun, checking the bore as I always do. A ringneck flew up and I touched the 870 off. I was greeted was a big explosion at the end of my barrel and the bird flew away.
Barrel was peeled back about six inches. I contacted Remington and they said "Send us the barrel". I di dand they said that. "There was a metal obstruction in the barrel or a stone as the inside of the barrel was scratched".
They sent me a new barrel and about two weeks later a gunsmith told me that there was a recall by Remington due to faulty steel in some 12 guage barrels. Go figure!
I'm sure it was the mud in the barrel that made it blow but the man was an idiot as well. His idea of coon hunting was to run down the river and spotlight the bank and rob someone elses traps. Like I said I never went with him again.