Your hose bibbs are very vulnerable to freezing. They get some warmth from the walls of your house and the warmth of the pipe leading to them. If the water in them freezes it can expand and cause them to break.
Inexpensive foam covers that attach to the knob on the bibb can be purchased at any hardware stor. This is a very simple way to help protect them. Make sure you buy these before cold weather sets in. They may not be available when everyone rushes to the store to buy them. In a pinch the hose bibb can be wrapped in a towel or blanket.
If you have a valve that separately turns off your hose bibbs that can be a good way to protect them. Remember though that turning off the water is only the first step. There will still be water left in them. If your plumber installed a valve to open up the system to the air you may be able to blow them out with a compressor.