they apparently were not quarantined long enough. Either that or they weren't soft shells at all and were diseased hard shells. Sulfa blocks can help prevent this sort of thing and cost about 5 bucks. Sulfa blocks last about a month.
Unfortunately in the pet trade thier shipment conditions can be quite inhumane and unhealthy. Here in the states we don't see it much and I would expect the EU and UK to be even more stringent but I am not certain. in places where regulation isn't strict many don't make it much past shipment that store may want to check out thier supplier. It is too bad about the turtles.
However, mine are going to end up in a yard water garden I'm buiding next spring. Turtles are a lot of work to keep clean and free of odor in an aquarium. Outside they will have a pond to live in. and the biofilter is robust enough to keep them clean.