The bad blood started long before ATT purchased Directv or Time Warner.It is all speculation, of course. However, there have been some pretty solid signs that Dish is heading towards a major dispute with the NBC/Comcast channels. (NBC Sports Chicago is already long gone, since fall 2019.) So, if/when the remaining NBC Sports channels get dropped, that would only leave the AT&T Sportsnet channels, for however much longer their contract lasts. Dish could renew with AT&T for continued carriage of those channels, but why would they? With all of the other RSN's (the vast majority of them) gone, and considering the long-standing bad blood between Dish and AT&T over HBO and Cinemax, it is easy to imagine Dish dropping those channels also, and being completely done with RSN's. This would help make Dish's pricing consistent across the entire country, instead of needing to add Regional Sports surcharges in select areas. Dish has been reluctant to add surcharges for more expensive local channels markets. So, it would also make sense that Dish would be looking to eliminate the (relatively recent) Regional Sports surcharges in the few areas that have them.
Remember, early 2000’s ATT and Dish were partners in a telco situation, and Dish ended up backing out towards the end. ATT was in talks of buying Dish, and Charlie told them to kick rocks. At the time, the CEO said he would never work with Charlie Ergen ever again, and the beginning of the blood feud was born.