I'll start by asking some follow-up questions. Is it possible that one or both panels were glued to your work surface? Any signs of defects in the wood? What thickness ply were you using? What quality? What type of scarf joint did you make? 8:1? Something else?

A 17' long piece of 4mm ply is pretty floppy. Rather than lifting it flat, I'd turn it on edge and maybe get a helper to maneuver it around until assembly gets far enough along so that the panels have some shape to give them stiffness.