Don’t know, @PeachyDaisy. They acknowledge that as a limitation and are looking into it, but I think we can mull over the possible explanations ourselves.
If you believe other people must see or refer to you in a particular way, it’s conceivable that this can make coercion or even violence seem more justifiable when people refuse to comply.
There are also online trans communities where violent rhetoric is sometimes justified or encouraged. Being immersed in that sort of environment could potentially reinforce those attitudes and be a contributing factor.
This may sound harsh, but I also think some of the rhetoric in these spaces can be remarkably detached from facts and reason. Even asking what someone like Widdecombe has done can produce bizarre responses. I do wonder whether an ideology that places subjective identity and feelings above biological reality could contribute to that mindset.
I’m not saying any one of these things causes supports violence, but there seem to be quite a few plausible mechanisms that could potentially be causative, although there may also be factors that are correlative.