I dunno what to say. Is the weight supported on that plywood side, and the 4" wide plywood strip that is running vertically at 90 degrees to the side? Or is there a post hiding behind that corner.
I'm assuming this is built like the Oceanic stands I've looked more closely at, and there is no post back there. In that case I'd definitely draw it back together. Liquid Nails and screws with wide heads or washers. Counterbore (pre-drill) the side piece (where the head will sit) with a hole that the screw can slide through fully. (I wouldn't use drywall screws, as they might split it. If you use flat head screws, countersink them. Round head screws won't be as pretty, but they might be strongest.) Start at the top and work your way down.
Another option would be to use 3/16" carriage bolts, drill all the way through the side and the upright support, then gradually tighten them all to draw the side back towards the support. I'd also check the front corners to make sure something similar isn't going on.
Nate