It'll be a legacy "County reject" tradition from the days when loyalty still existed in the game. I couldn't understand it when Tommy Johnson got booed on his return with Derby after all he'd done for us, but it's just one of those things we have to accept as "part and parcel". There's always going to be a sizeable proportion of fans who will have been let down by somebody at some point in their lives and things like this can act as a trigger.
Reminds of me rock drummer Ginger Baker who apparently couldn't handle goodbyes after he waved farewell to his father at the train station as a young child during the war and never saw him again.