Perhaps it was the realization that she didn't actually believe what she claimed to? It did sound a bit odd to me when she said it, for what it's worth.

To be fair it does seem to be consistent with the "vegan intuition"---whatever we call the deontological or virtue ethic that tends to drive the bearer to an 'all or nothing' avoidance of the animal products. It is only inconsistent with the approximately consequentialist justifications often given for that value system. That is, the abandoning of the veganism isn't much more absurd/weird/distorted than what drove the veganism in the first place. It just isn't an ethical system that happened to have been based on Von_Neumann–Morgenstern axioms.