
Several reasons IME...
Mucky dogs left without the muck being brushed out, collars left on and not brushed underneath them (matts up my dillydogs neck a treat that does!), other dogs saliva, that orrible spitty sticky mess they get after eating rawhide chews, thats like glue....
Other causes are not brushing out loose undercoat, plain not brushing right down to the skin, clippering coats that are supposed to be long - thats because the guard hairs and undercoat end up all the same length and one gets tangled around the other.
Biggest cause of tangling and matts..... the morning of an event for which you require your dog posh and clean and neatly brushed.... (take one neatly groomed little lurcher to a photo studio - retreive from car one small grey matt with legs and eyes.... somewhere.)
Em