As posted by Fabrizio, Nottingham Forest has parted ways with it's manager Nuno Espirito Santo, these's was a tense situation at the club between him and them, I believe that's the main reason why he was sacked. Nottingham Forest has started off this season with a win, a draw and a lose in three matches. I believe that's one of the major trigger as to why he and the club management has fallen off and it resulted in a sack.
(Source)The reasons for Nuno Espirito Santo's dismissal are quite complicated. It is possible that the tension was the result of Nottingham's poor performance under their former coach. And it seems that this tension began last month, where Nuno Espirito Santo realized that his position at the club as coach was under threat. But it seems Nottingham Forest are moving quickly to find a replacement for Nuno. And they have appointed Australian coach Ange Postecoglou as his successor according to available information (Source). However it maybe there will be a major change in Nottingham's performance after their coaching change. So let's see if Nottingham can perform well with this coaching change or not.