The long sides are held by bolts in a typical flat pack furniture joint. This is normally fine and strong enough, although ideally there should be a half moon washer/nut instead of just a regular hex nut.
However, the bolts are too long, bottoming out in the hole and forcing the side and leg apart instead of clamping them together.
Fixed with a long drill bit, making room for the tip of the bolt.
Must've come from the CBA furniture factory! :)