Assuming you're talking about the clear plastic braces that go over your teeth (i.e. invisalign), then yes they increase the risk of cavities. During the night they obviously cover your teeth, which is ~7-9 hours (or more if you're a degen) where your teeth aren't going to be in contact with your saliva, basically emulating the effects of drymouth. Saliva washes away food and plaque and helps to keep your teeth a little bit cleaner. Not to mention plaque would undoubtedly get stuck in the hard-to-reach places of the retainer, and plaque being pressed against your teeth in a dry environment every single night = cavities.
Keep trying to be an e-expert of everything Xant, it's fairly amusing.