What constriction were you using on your screw-in choke? It's generally safe to fire most slugs through even tight fixed chokes, but if you have the option of changing them, you should go to a Cylinder or Improved Cylinder. Otherwise it can cause stresses at the transition between the two parts, and in the threads. Or were you just using the ported tactical Cyl choke?
Was the choke turned all the way down, tightly, or did it come loose?
How many slugs are we talking about?
The Hornady SST slug is a sabot-type, for use in fully-rifled barrels. You weren't using a rifled choke adapter? Pretty pricey rounds to be flinging about like that. Still, the projectiles are a significantly smaller diameter than the bore, and the plastic sabot fills up the barrel, so I don't know that it should be pushing too hard on your barrel. Maybe part of the sabot got stuck, and caused a blockage for the next shot?