Well from the little information circulating and got sure too, I can remember last season when Nottingham Forest owner stormed the pitch in a match and was furious with the display of Nottingham Forest players. I knew there was going to be a problem later, it seems like it was that incident that brought a bad blood between the owner and the coach. It's just my observation from last season.
Your observations might be right dear, although other situations must have caused the disagreements between the coach and Evangelos but if he held grudges just cause of a poor performance in one match then he could've sacked the coach immediately after the end of the previous season than waiting till the beginning of a new season just to sack him after three matches.
It's so humiliating for a coach that's on a good track, Nottingham's performance this season is not bad so far and since the coach has already had pre-season trainings with his squad the owner would've allowed him continue his job, it could even be frustrating for Postecoglou the new manager in a case where the new players Nuno bought doesn't align with the quality of players he needs. Well, Nuno Espirito showed great potential so finding a new job won't be difficult.