> do people violently oppose vegansI doubt being vegan is the key here.
Though there is a fair amount of bullying, through ad-homenim attacks or idiots just trying to wind people up for shits & giggles. There are also some with a persecution complex that seem to take even the slightest criticism as a violent attack. This is not constrained to veganism though: there are similarly unhinged people in many walks of life.
> or animal rights activists?
I expect this is the more important factor. Being a vegan is likely a consequence of her strong feelings in this area (the fact that some are highlighting the vegan thing is an example of the bullying I mention above). Animal rights is a very emotive area and some people (a few, but a very active and vocal few) get dangerously passionate about it to the point of losing all view of any other logic or moral sensibility (like not killing human animals).
> a workout video being age-gated: who would oppose that video?
I've not seen the video in question so I can't make specific guesses, but there are a couple of possibilities that spring to mind:
* Some people are very puritan about the showing of skin and so forth, perhaps there was a complaint about that sort of thing?
* Was anything potentially divisive said during the workout clip, even in passing? Given her apparent anger at the world that is not unlikely.
* It could just be someone that disagrees with her views on something else. Down-voting and reporting entirely unrelated content in response to something else is an immature but very common reaction.