One of the reasons is because they are a huge part of the reason some people are aware of what Viv has done and are known in the opposing side for Viv. They have brought a large amount of attention to her wrongdoings and have many eyes on them as a result. They have made people aware of Viv's transphobia, Viv's immaturity, her thin-skinned attitude, and do on. This has led many on Viv's side to deem them a stalker and a hater even though most of the information they get on Viv is either public knowledge or given to them by Victims of Viv.
The second thing that many have disliked about Chai were their interests. They have called Chai many things, but one of their main gripes was Chai's ships. If I am correct, they dislike the Hiccup x Toothless ships mainly, calling it Zoophillia. Chai has been on record stating that they only ship the two with Toothless thinking and feeling like a human and not thinking and feeling like an animal. I personally am not a fan of the ship, but the accusation can be much imo. I just find it odd. Maybe they know something I don't about Chai? Tbh, I kinda am outta the loop in all the deets regarding that, so I would def recommend looking into that and seeing for yourself just in case but I'd rather stay neutral on this in case there's just something I am unaware of, but as I stand, the accusation seem harsh. People mainly ever bring up the Hiccup and Toothless one, and some others bring up other things like Chai's art and accused them of being a pedo (his character is an adult I think but some art of his character looks chibi so that's where the accusations began I believe) and something about Pokémon and Transformers?? I think transphobia for the latter. Chai is trans btw and I think they simply didn't like how one Transformer was forced into being trans)