Cabinet side separating - how to fix?

We bought some cabinets from Home Depot about 8 years ago and had them installed. Recently the top shelf in one of them kind of tipped. I thought the little pegs needed to be replaced but after further investigation I found the side of the cabinet is bowing outward so that the pegs on that side don't stick out far enough to adequately hold up the shelf. The back and side panels of the cabinet are separating; the back appears to normally fit into a groove in the side panel. Here is the back corner of the inside of the cabinet: Attachment 39964 (http://www.doityourself.com/forum/attachments/carpentry-cabinetry-interior-woodworking/39964-cabinet-side-separating-how-fix-2014-10-12-0000.jpg) A close-up: Attachment 39965 (http://www.doityourself.com/forum/attachments/carpentry-cabinetry-interior-woodworking/39965-cabinet-side-separating-how-fix-2014-10-12-0001.jpg)