It's something I wonder about too, not so much in terms of hardcore AR activists (I have a couple of theories about hardcore AR activists who aren't veg*n) , but in terms of people who are very passionate about animal welfare, so much so that they spend a large amount of their time and resources doing rescue. I think for them, at least, it comes down to a weird inner bifurcation, the same one I had for my life before going veg, a bifurcation which is the norm in our society.
OTOH, for me it is equally difficult to understand how so many veg*ns can fail to take proper care of their companion animals, or walk away from someone being cruel to an animal without doing anything about it other than complaining about how much the sight hurt their own personal sensitivities. Actually, that's harder for me to understand, because by becoming veg they have (one would assume) already gotten beyond that bifurcation.
As for hardcore AR activists who aren't veg*n - I think there are certain individuals who are drawn to the mythos surrounding *revolution* (for want of a better word) because it fulfills a need in themselves. They don't necessarily believe in or care about the principles which are being fought for - they get their kicks from the fight itself. I think you find those kinds of individuals in any movement - revolutions to empower *the people* where some of the revolutionaries couldn't care less about the good of *the people*.