Briefly: The hate started when the 'real' Ronda Rousey was exposed on TUF in late 2013. No amount of editing and PR could cover up who she really was.
Throughout 2012 and the first half of 2013, most hardcore fans loved her. She was bad ass and a breath of fresh air. But on TUF Ronda was exposed as an immature, bullying teenager surrounded by sycophants like Edmund. The hardcore MMA community turned on her. When she fought Miesha at UFC 168, she was booed before and after the fight.
Thereafter there was a concerted attempt by the UFC (and presumably her talent agency, WME, which now owns the UFC) to cultivate a more positive image for Ronda. She was constantly in the public eye, but this time in carefully managed situations in which the 'real' Rousey wouldn't come out. This helped her image considerably. In 2015, she even went babyface after Bethe Correira made some ugly comments that seemed to reference her father's suicide. She was depicted as a superhero and, gradually, even the hardcore fans who disliked her began to come around.
However, at this exact time Rousey began to appear increasingly hubristic due to her success in and out of the cage. Ridiculous hyperbole about her being the greatest female athlete ever annoyed fans. And at the UFC 193 weigh-in, Ronda had some kind of emotional meltdown, and she has never looked the same since.
The rest is history.