Injuries to our first choice centre back pairing plus the integration of a new centre half might well answer all those questions and even some you haven't asked yet!
Certainly no coincidence that we've suddenly looked much more suspect defensively without Kompany and Mangala.
Questions of tactics and formations have been done to death, but for me, it is, above all, a confidence/mentality thing. At the start of the season, we were defending aggressively, high up the pitch (think back to how often Costa and Hazard were tackled around the halfway line in the Chelsea game), and so there was little space between defence and midfield. Without Kompany and Mangala, we've dropped off further, been much less aggressive in our defending, and so that gap between defence and midfield has reappeared. Even with Fernandinho and Fernando in the centre of midfield, it makes little difference - if we continue to defend like that, essentially every team in the league has the potential to cause us problems, as we saw countless times last season, and as we saw again on Saturday. Hopefully, the return of Kompany and the ongoing integration of Otamendi will see us get back to how we were in the first few weeks.