The first one I can think of definitely knows her reasoning: she doesn't like the taste of meat (though she is more of a vegetarian). For her to eat meat, it basically has to be overly processed to the point where it really doesn't taste like meat anymore (e.g. she will eat hot dogs and pepperoni occasionally).

For the 'true' vegans I know, I'm pretty sure they don't know what their reasoning is, and the only thing that would need to change for them to consider eating meat OK would be for it to stop being trendy for them to be vegan. At least, they've never been able to clearly articulate a position to me.