No and yes: it's caused by the amount of torque you're putting into your cranks which at the moment likely exceeds anything you could create on the flats, never mind being static. It is highly likely that you are also overpowering your stoker's efforts which is related to power differences, but not in and of itself the reason the top run of your sync chain is under max load: that's all you.

As for where all of that excess chain comes from, a lot of it comes from the chain's own elasticity, some comes from bottom bracket spindle deflection, and tandem frames will also deflect a bit under torsion loads and a Calfee is not immune from that either... but I'd bet your eccentric probably slipped a bit on that climb and that's where most of it came from.