It's partially an abrasive thing, but it's also the ability of the barrel to physically withstand the outward pressures that steel shot places on the barrel.
Lead shot compresses both as the shot is fired and as it moves through the choke, so the stress on the barrel is relatively minor.
But steel shot doesn't compress. That's why steel shot is loaded with buffering material, and loaded into a pretty thick cup wad. Because the steel won't compress, you have to provide padding that will compress.
Even with the buffering and the shot cup, it's not uncommon for steel shot to chew right through the cup.
In a new shotgun made for use with steel shot this can eventually score the barrel.
In an old shotgun, the barrel can rupture anywhere along its length. Most commonly, though, the failure is at the choke.
