Kitchen cabinet coming away from the back of cabinet, help?

You should reinforce it. Here's one way to do it. Glue and nail a piece of 1x2 to the back piece. Then nail the side to the 1x2. Run some glue on the side panel before nailing the side. If that is not able to be done, then glue if afterward. As far glue, get a product called PL400 construction adhesive. It's by the caulks in HD/Lowe's. Use #4d finish nails for the nailing. Now the only thing is you need to do one of 2 things. Either notch the shelf for the 1x2's installed, or put in pieces of 1x2, so they don't interfere with the shelves. I would notch the shelves. Hope this helps