I would hazard a guess that backend upgrades made keeping your plan as-original became impractical if not impossible, but hiking your price up and losing you as a customer was also wholly unappealing because any money is better than no money.

So presumably AT&T did the next best thing for both you and themselves by silently(?) upgrading your plan to whatever was the new minimum of the new hardware while keeping true to the price lock to keep you as a customer.