Rover used to work on the trains, welcoming new players to their chosen city. Then after 2002, he decided to settle down. He became rich and famous and had an illustrious career but he was seriously in debt to Tom Nook. So, Nook hired a hit man to kill him.
After that Rover's ghost returned to the trains, to continue what he had begun in 2002. Tom Nook didn't want to be caught, so he went into the property market, after paying a large, undisclosed sum of bells to the Mafia Boss Lyle, to protect him from Copper and Booker's investigations.
I'll see myself out...