The wood has split under the backbox on both sides. The weight of the backbox is making it worse and pulling the back up on the same piece of wood. The wood cannot handle the weight and is going to cause the backbox to sink into the body of the cabinet. It's not going to be pretty when it happens. My plan of attack; 1) remove the entire backbox to get a better idea of how extensive the damage is 2) minimal damage and I'll try to glue/clamp the wood back together. 3) max damage and I'll remove the wood entirely and replace the piece 4) glue, fasten, sand, smooth, prime, paint, sand, final coat. Please see the pictures.