The brown along the break indicates previous weakening from corrosion. Been there for awhile. Previous firing with corrosive primers gets into seams on the twists, doesn't get cleaned out with bore cleaner.
Not even likely since the fracture doesn't follow either the weld joints of the ribbands, or of the Damascus pattern. I'll say this was due to high pressure. The problem is to determine what caused a pressure spike that ruptured the barrel. As for the rust, bare clean metal can have this amount of rusting overnight under humid conditions. We need to know how soon the picture was taken after the burst before we even consider that rust staining as significant. And how many guys at the gun club ran their fingers with salty perspiration across the fresh break? It would be nice to see if the primer of the ruptured shell is any different in appearance than others from the same box.
