Matting is caused by a number of things.
1. Improper brushing. There is a right way and a wrong way to brush certain types of coats, as well as the correct tools to use. Try the internet or a breed specific book for starters, or your Groomer or stylist will be happy to show you how to brush and the proper tools to use on your pet.
2. Bathing: If you bathe your pet at home, no matter what product you use, if the hair is not COMPLETELY COMBED OUT before and after bathing, any mats or tangles can be worsened by the wet/dry process.
3. Shedding: "Nonshedding" breeds such as poodles and shitzus really do shed (just much less), however, the dead hair gets tangled up with the live coat, and if not brushed out regularly, it will eventually form mats.