Looks like it was a B739. I fly 737s and a -900 is limited to 25,000 on a single pack, so it does look like it was some sort of pack failure/overtemp/overpressure that wouldn’t reset, so that’s the likely reason for the descent.

You are lucky that on a flight of that length it didn’t divert due to the increased fuel consumption for the lower altitude. Dispatch must’ve been able to run the numbers and confirm enough fuel onboard. Always good to have extra gas!