In my opinion he's just on an extended run of scoring mostly relatively simple tap ins each week, his overall game has hardly improved.
Without checking I believe he's still only got 3 assists in the PL all season.
He's not exactly ripping defenses to pieces for 90 minutes week in week out, scoring high quality goals, creating multiple chances or generally contributing much to the overall team performance... he just keeps popping up in the right place at the the right time once in each game to tap in a sitter, usually as a result of united sucker punching the opposition by hitting them on the break after being outplayed for long periods off the match.