Could have been just one rubber washer in the left wheel cylinder which didn't seal. The other extreme could be that brake fluid can only handle a given amount of moisture, and that amount had been exceeded. More air could be drawn into the system as you drive. But, since you don't drive the car much, there should be very little moisture in the brakes. In storage, the temperature will rise and fall daily, possibly drawing some moist air into the system.
All this considered, I think it was the defective wheel cylinder or washer. Glad you have fixed the problem!