It’s much more like politics than it is religion. People take a stance because of their values, ethics, mortality, etc (as opposed to religion which is generally a more irrational basis for belief). And the reasons for being loud about it follow from that.

So if you can understand why someone might proselytize about the situation in Gaza, then you should also understand why someone might proselytize for veganism, even if your views might align differently for different issues.