Look's to me like it's been done while you were last out. Who was out with you and were they stood at the back holding on etc, I don't think that it has been done any other way. You could try and take it off and straighten it out but the tubing's probably weakened so it would bend again. I'm sure some steel fabricators like martini on here can give you some better advice.
the only person who sits behind me is my 7 year old daughter. I would hope the S/S is stronger than to bend at the pull of a child. She was siitting on the rear jockey, feet on the footblocks I've made, and holding on - not standing with lots of weight on the handhold
Also the last time out was flat calm. If I'd had a heavy crew member behind me in heavy seas I could possibly understand it.
Seriously what sort of force would cause that damage?
Obviously that depends on the grade of SS used. There is a LOT of leverage available to multiply force on that fitting. The fact that the GRP didn't get damaged suggests that the forces weren't enormous? You could try a pull on the top of the navigators seat and see what happens - if it bends, you know you're changing all four anyway
Like willk says looking at the pics there seems to be a lot of leverage from the fixing position up to the backrest and although your daughter is only 7 if she is say,, 5 stone, that's 70lbs so when accelerating that can still put a lot of stress on the backrest. I'm not saying that is the cause, but just looking at all the possibilities. I don't know how old your rib is but it is a ribcraft according to your profile and I would have thought that there fixings would be of good quality.
I was thinking more along the lines of not blaming the pups and asking the big dog did he "rest" his posterior on it...
or has the boat ever been tied up to the back of the seat - I've seen people doing this to keep the ropes off the tubes. Its hard to imagine a way that that sort of damage could happen in storage with a cover on - without causing other damage to the cover or boat.
Is it reapairable or do I need a new backrest fitting?
It's repairable. I personally would remove from the seat pod to straighten. Place on a block of wood and use moderate force and lean into it. Check for creases in the stainless-steel. If you're in any doubt - I'd replace.
It will repair easy but it's happened once it will happen again. Most stainless guys use 1.5mm wall tube and it's getting difficult to get thicker stuff
The trend for those type of backrest is to use 32 instead of 25 mm
You're hoop looks flat across the top so the 32 mm former radius can cope with that width of backrest