Simply they have been worked out. The CB's have been identified as slow - same with Ivanovic - Costa has been worked out as he can't take what he dishes out and isn't really able to dish it out now - Fabregas has just started where he left off last season - wank - for some reason Hazard is a pale shadow of last season but to be fair you can't carry a side on your own and most tellingly they haven't strengthened so there is no pressure to perform and no second string to reliably fall back on.
Throw in that mix the issues with the lovely Eva - Courtois in the news for the wrong reasons and what could be a soured manager/chairman - a back room / dressing room saga probably like our first defence of the PL title one and thats what you get - an out of form unfocussed unfit squad of players who are struggling to get their shit together