My first thought is I hope you are changing all four tires, not just the rears. The most common cause is corrosion between the wheel and the hub. It is more of an issue when alloy wheels are mounted to steel hubs. This sets up a condition called galvanic corrosion.Apply a liberal dose of penatrating oil (PB Blaster is my personal favorite) If you loosen the lug nuts several turns and rock the car forward and backwards. Something in the area of 3-4 mph forward then a quick stab on the brakes, 3-4 mph backwards then a quick stab of the brakes. The idea is to use the enertia of the car to break the wheel loose. Avoid usung a mallet on the wheel as you could damage the wheel or the tire. Once the wheel gets loose apply some Never-Seize to the hub and the inside of the wheel. This will prevent them from locking together next year.