Periodontal disease. Teeth are rooted in the gums, so any gum disease, most common is inflammation known as periodontosis, will weaken the gums and will contribute to teeth loosening. It is very important that you not only brush but also floss your teeth at least once daily. Also, to prevent inflammation caused by acid producing bacteria in the mouth - use xylitol containing products (gum) after meals.
Loss of bone. Bony support is required to keep teeth stable. If bone is lost, via periodontal disease for example, teeth can become loose.
Trauma/cyst/resorptn. In addition to periodontitis (gum disease) which is certainly more common, trauma to your jaw could break either your jaw and/or teeth resulting in loose teeth. A cyst, tumor or cancer could be growing in the jaw causing teeth to become loose. And there are different kinds of resorption that can occur: external vs. Internal, that could contribute to adult teeth becoming loose & eventually lost.