He now claims that it was never their 'forever home' and he always wanted to sell it within a few years. Not true. When they bought it, like you said, he was making long term plans. He even mentioned Emilia getting married there, as the previous owners had a wedding there, too.
So yes, they can't afford the mortgage, or have realised that the house needs a lot more work and they can't afford that.
Just found the neighbour's house. Very nice. Open, bright.. I bet they don't have a chimney full of dead birds or wasp nests in their loft.....