Dogs that have longer coats can become very susceptible to having their coats mat up. There are a few things that can cause this but a couple of things are very common. Dogs that wear collars constantly can end up getting matted up hair on their necks around where their collars rest.
Dogs also get mats in their fur from friction. Dogs might go out and run through weeds or bushes or they might flop down in the grass and roll around. In turn they may flop down inside and roll around as well. All of these actions give the dog a chance to get mats in their hair. You can use special brushes to help get mats out of your dog’s hair or you can have them groomed either professionally or at home yourself.