Do you own a cat:

Seriously, how was the film stored both prior and after shooting with it? Cold, frozen, warm? If stored cold, which roll was used first? Was one roll in a canister and the other not? Did one roll remain in the camera? Was one roll kept warm in a pocket and the other not?

Typically, going from cold to warm will draw moisture from the air. For example, when you take film from refrigerator or freezer, and then pop it out of it's canister in a warm, moist environment. IF this happened with the roll with the moisture, while the 2nd (dry) roll warmed up still sealed in a canister, that might explain the difference.

My primary concern would be the possibility that the film was moist or wet (inside the cartridge) prior to using and that some of that moisture transferred into the camera. The leading edge of the roll could have been dry, so you wouldn't notice it when loading. As a precaution, I would store the camera exposed, with the back open, in an area with good, dry airflow.