I had this issue out of the box. Then when I transformed him to space ute and back it was okay. I think it's the angle of the chest. If you angle it forward a bit more so that the top of his torso is sloped downwards then all the linkages to his back panel change slightly and the tabs on the back stay in place better. I believe that's what's happening.
Gee it's a clever transformation how they get a deluxe figure to go from a rounded robot to a boxy vehicle with minimal kibble or visible parts in either mode. I've been amazed at all of the transformations of these Studio Series 86 toys (I'm including Cyclonus as well). They've got some very talented designers.