The first few archwires are thinner and flexible. Because we chew hardest with our back teeth, that's where food presses on the archwire the most. The stretch of wire between the last two brackets is the longest, because these are the largest teeth in the mouth. The longer the stretch of wire between two brackets, the softer and more flexible it is, and the more easily food bends it. That's why you may sometimes notice that the archwire by the back teeth has bent towards the gum in a U shape. As I mentioned, these archwires are very flexible. If you press under the curve of the wire with your finger and wiggle it gently, it will straighten out.
This answer is for general information. Your own case needs an examination by an orthodontist.