Mossberg 500 Muzzle Cracked Open

I recently purchased a used 12 gauge Mossberg 500. According to the serial number the gun was manufactured in 1990.

Last month I finally got the chance to take it out bird hunting. Since I hadn't shot it before, I took it to a secluded field and got off a shot. Everything appeared to be working so I figured I would be good to go to hunt with it for the day.

While hunting, I shot two birds and missed another. Upon dispatching the second bird, I glanced down at the gun and noticed that the muzzle had completely blown apart (see image).

So my question to you folks is: What could have caused this? My leading theories are as follows: 1. We were crawling through heavy brush and picker bushes throughout the hunt. At some points I had to place the gun down to crawl through the mud. I noticed some mud on the muzzle (not in the barrel) which I wiped off. Could some mud have gotten into the barrel and caused an obstruction? 2. The barrel deceloped some microscopic cracks in it that weakened it and caused it to split open. 3. The choke was incorrect for the gun/ incorrect for the shells being used/ put into the gun incorrectly

The real puzzling thing to me is that I got off 4 shots before the barrel was destroyed. Any help that you folks can provide is greatly appreciated!