Did you ask him why?
I can think of a couple reasons ... if he was the breeder and was running on more than one, he might have decided the other one was his “keeper”. Or if he wanted a breeding prospect and something came up that would rule that out with this dog ... or maybe he really, really wanted a different breeding and just got the opportunity to get one from that. Or maybe he just likes starting young dogs more than training more advanced ones ... or maybe he’s looking to make a profit (depending on how much he put into the dog and what he can get for a started dog). Or maybe this dog’s temperament isn’t quite what he prefers to work with (although some might like it).
I don’t think it’s something I would do necessarily though ....