If the air pressure inside the tire drops then the bead will slip off the rim, and you lose your liquid ballast, especially in the winter when the cold temperatures cause tire pressures to drop. Remember, heat makes things expand, and cold makes them contract.
There are a couple of things that I have found to help; one is to replace the rubber valve stems with metal valve stems, and the other is to sand the bead area inside the rim with crocus cloth to break the smooth surface that the powder coat has.
Put the valve stem at 12 o'clock and check the air pressure in the tire and rim before you go out to do work. Taking a few moments to do this can save you a lot of time and money. I assume that you do have an air compressor to add air to the tires.