Why did Chelsea players down their tools?
There's a huge misunderstanding about what transpired that season. I'll try to clear up some of the confusion. This is how events transpired:
1. Mourinho drove Chelsea to the title in 2014, clinching the league with three games to spare (a 13 (!!) point lead over second-place Man City after 35 games).
2. Mourinho, just like Antonio Conte this past off-season, recognized his team needed reinforcements -- new players to add to his squad. The board said "no." Mourinho then spent the entire pre-season telling the press and his players they would need to be even better individually than the previous year in order to be successful.
“It’s a big challenge. To be better with the same people, the players have to be better individually than they were last year. So when they think: ‘Oh last season I did great,’ this season it is not enough,” he added.
“John Terry – great season. This season it is not enough. Must be better. Fàbregas – I don’t know how many assists last year. Fantastic. It’s not enough. Because the others are going to improve with the players they are bringing and we have to improve by ourselves with our work.”
3. Mourinho allowed his players to monitor their own fitness levels during the off-season (so as not to drive them too hard and burn them out). The players failed miserably, coming in out of shape. And the poor results followed. The players knew it and said as much during post-match press interviews, including after Mourinho's sacking:
“I sent [Mourinho] the [text] message to say I was sorry he had gone and … well … just that I was sorry,” says [Hazard]. “We’d enjoyed all that success together last season, but this time round we hadn’t. I felt a little bit guilty because I’d been player of the year. I’d been one of the most decisive players, and this year I’d performed less well."
Note: Hazard had a 25-game scoreless streak to start that season.
4. Eva Carneiro. Wow. That situation spiraled out of control with a fury, and pushed some of the players over the edge (namely, Hazard).
5. Lost to newly-promoted Leicester City (before anyone believed they would win the league). Sacked.
Something to note: Mourinho never lost the fans in spite of it all. WAGNH had a poll after the Leicester City game, with thousands of responses: sack Mourinho, or keep him? 84% said "Keep." One point clear of the relegation zone, and 84% of the fans still backed him because they knew the fault lied with the players.
"Even when we have lost matches at home the supporters have chanted his name, there has been very little dissent about him going [wanting him to go]. I would have thought there would be frustration with some of the players." - The Chelsea Supporters Trust
Hopefully that clears up some confusion.