It's pointless to speculate what happened. It's definitely not a coincidence that all players suffered a big loss in performance levels (Willian being the only exception). Something caused it, it's not possible for us to know what, but for me the most likely explanation is a combination of things: short pre-season resulting bad fitness; season starting on a negative (Evagate); consecutive bad results snowballing and affecting confidence; possibly some of them disagreeing with Jose's methods.
I think these are the most likely explanations for their fall from grace. Hazard and Matic don't seem to have the mental strength to recover, while the likes of Fabregas and Costa have it and have recovered. These four were our most important players last season along with JT, so it's no surprise we were relegation fodder at one point. With ~half of them having recovered, we're now 10th - seems about right. We would still have needed Hazard, Matic, JT, Ivanovic and Oscar to reach their true levels, but I'm afraid it hasn't happened, so we are where we deserve to be.