I would say it's your FAT arse that's caused it to bend
It happened on my MK 1 seat back and cracked the weld so that it fell off and ripped my seat cusion
MK 2 is looking rather good however
I think you'll be surprised at the sort of loads a seat back takes as you try to brace yourself for impacts.
Either repair and brace the exsisting seat back by running either a stainless fillet down the bent tube/welding another tube from the grab handle to the base of the seat pod, or remake it with heavier gauge stainless tube.