I'll answer once for all those who asked.
De Rossi's dismissal came as a surprise for everybody. Rumors had circulated about him having a fight with Cristante during training.

According to some journalists his relationship with Lina Souloukou was strained to say the least. But nobody expected he'd be out so soon, even after a bad start. Last two matches were unlucky but well played overall, so we expected better performances over time, considering many players are new.
They decided to sack him in the middle of the night. It's highly likely something we don't know happened.
In any case, they've made a huge mistake. It remains to be seen whether they did it in may when they extended De Rossi's contract - 3 years for 3 million euros a season - or a few days ago when they sacked him.
They also underestimated the fans' emotional attachment to De Rossi. Most of the hardcore fans lost their minds, threatening Souloukou who's been put under police escort and decided to resign shortly thereafter in order to protect her children.
Now they need to find a replacement.