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Ben, my guess would also be an over/double charged round. This would produce an over pressure spike that would do the type of damage you report. I would see what Sug has to say and hopefully they will do the honorable thing and replace your pistol. I'm glad you were not seriously hurt in this ordeal.
its also possible that you missed the 3rd Squib round and fired the next round if you were using reloads and admit it to sig they will (most likely) say that your warranty was no good. If you are a reloader you NEED to look at your process 2 squibs in 3000 rounds I have had 1 squib in way over 100,000 rounds loaded and that was when I started loading 35 years ago