Probably accident damage and poor repair...
Quite possibly, it's a vulnerable spot. Depending upon what they find when they strip it back, a permanent repair may be possible - but if the rust is in the inner/outer panel seam, it's going to need more work than a panel and paint shop want to do. Everything can be fixed, if it's done properly.
A new bare tailgate in primer will be a small fortune, and then you will have to pay for painting and glass fitting, plus the job of actually swapping everything over. That's going to add up to a big chunk of the value of a 2010 car.
Personally, I would be going for a repair, or a used tailgate in colour. Having a quick flick over ebay tells me that you're looking at £200-£300, those prices on there are not usually far out... sometimes cheaper than a breaker, and you can often barter a bit in person. If you can find an advert for a car breaking in the same colour, you can sometimes get a better deal.
