Marvel: Contest of Champions players are upset with Netmarble after the company acquired the game's developer Kabam. The uproar came after Kamam issued an update to the game that nerfed a number of heroes, and made other changes to even the playing field. The fans behind the "#BoycottMCOC Movement" have even sent out a press release, which you can read in its entirety here.
Kabam did apologize for the changes and a new update is in the works, but some fans do not believe they will address all the problems. You can read more about the MCOC controversy here, and learn why some fans are skeptical that their demands will be met.