Like everyone else its allergies and it can be to anything just like people. Our boxer it took us 5 different foods and finally blood tests to determine what he was allergic to. His allergies are way off from what other dogs allergies are, he's allergic to milk, eggs, rice, oats, duck, barley, catfish. So we had to put him on food that is completely grain free - that can get costly but we finally found a food for him where he no longer scratches or chews his paws non stop.
Blood tests for allergies can be pricey ours was %200 but at least we found out what the problem was I would suggest doing the same thing - get them checked