It's probably due to the historical reasons. Manchester United was always beating Aston Villa in away or home games in their last three games. It is most probably the main reason why we do see such very high odds for Villa. Even though their performance is still far above United, they can also drop three points in this game.
I think 2.20 is quite tempting for Villa. It's a home game, and they have a better chance to win.
Manchester United gives me feeling that they won't be able to stop their sinking ship in this match with Aston Villa. And a home away match brings more challenge to Manchester United while Aston Villa are in good form and very ready to host and beat Manchester United. First to say that they no longer want to stay under Manchester United like history shows, and second to fight for a position in top four of the league.