They don’t have the midfield for a back 4.First 8 games (back 3/5) = F8 A9
Next 11 (back 4) = F13 A20
This is perhaps making things too simple, (not accounting for opposition and lots more) but you can see that maybe they wanted to open up and try to score more as they weren't exactly banging them in. It hasn't really worked and they're just conceding more.