Sorry I didn't stumble on this one earlier...
The actual cause (drum roll, please) is the change in temperature in the water from the source into the home.
Cold water "holds" dissolved gases much better than warm water. This has always been counter-intuitive since warm water holds more dissolved solids than cold. But, I had it explained to me once by someone smarter than myself, the water molecule is more of a "V" shape when it is cold and the gas molecules get stuck in the "V". When water is warm, the molecule straightens out and the gas can escape. Most gases, especially oxygen, nitrogen, and chlorine are hydrophobic, or don't like being dissolved in water.
In the winter when water enters the treatment plant, it is cold and holds lots of dissolved gases. It is then pressurized and gradually warms up as it moves through the water mains and especially when it enters your house and water heater. Since the water is pressurized, the gas cannot escape. When the water comes out of the tap, suddenly all of the gases escape the water in tiny, tiny bubbles. The larger the temperature differential from the source water and the tap, the more bubbles you get. So, of course, this phenomenon is reduced or eliminated in warmer months when the difference in water temperature from the rivers and lakes and your home narrower. Most people with wells don't experience this problem because the water is not pressurized and is not exposed to as much air.
The gas bubbles aren't any chemical in particular, just atmospheric air. But, they can be dangerous to your fish as the bubbles are so tiny as to get caught in gill structures and cause damage.
The best ways to eliminate the gas bubbles have already been mentioned, either letting the water sit for while or running an air pump.