Snoke said he wanted to complete Kylo Ren's training at the end of TFA. Well, the last step to become a fully blown Sith is to kill your mate. (Like Anakin and Padme, the reason Anakin still has light in him is bc he didn't really actively kill Padme or something...)
The problem is: Kylo hasn't even found his mate yet. So Snoke waits for his next heat and then just lets every single alpha in the First Order have a go at him until Kylo mates with one. The whole process isn't very enjoyable for Kylo, as sex during a heat with anyone other than his alpha only makes it worse instead of bringing relief.
Finally it's Hux's turn and voila, he and Kylo mate. The only problem is: now Kylo is supposed to kill Hux to complete his training. But suddenly he really doesn't want to do that anymore, so they have to keep acting like Kylo hasn't found his alpha yet and is still in heat (heats without an alpha's aid can last up to a week) and of course he still has to have sex with everyone that Snoke sends to his room. But after a day or two Hux simply can't watch his mate be taken against his will by random Stormtroopers anymore, so he decides to flee with Kylo.