Thats an interesting connundrum. My only (wild) guess would be that the water got into the mast previously through a hole in the base during a full-turtle capsize. The salt or corrosion plugged the initial leak. Then, the mast heated up in the sun to create the positive pressure immediately prior to your drilling.

Like I said...its just a wild guess. I would try to find any remaining leaks by lightly pressurizing the mast with air and using soapy water to detect leaks.