Someone I know was bought out of the marital home by his wife who then put the house in her sole name. He then returned to live there and has been living there for some years but they're not a couple, but aren't divorced (apparently neither of them will pay for it). They've both had other relationships since too.
He does things to the house, and contributes financially towards the bills but not the mortgage. She's said that he thinks he's now entitled to half the house, should he decide he wants to go, or she does, etc. I think she wants to protect her interests. There aren't any children under 18.