Water often gets in tires by being part of the compressed air that we all pump into them, unless the compressor has a water seperator. The more humidity, the more water.
This definitly happens in more humid climates. When I was in the bicycle industry and ran a shop in the Wash. D.C. area we would regularly drain condensatoin from the compressor's tank, though it would take a lot of refills before a trailer tire had water from a compressor. More likely it's a flat spot from sitting all winter.
However, if someone wanted to play a nasty trick, they could pump water into the tires. We would sometimes use a hand pump to fill some unsuspecting employee's bike tire's with water. We had a guy commute to work at the shop for a week before he realized what was going on.
