Why do ex-vegans rail so passionately against veganism, and how can we stop them?

As Stephanie Ernst at Animal Rights & AntiOppression explains, there are three possible reasons someone might quit veganism:

1. Sheer selfishness (most common). These ex-vegans could manage to be (or pretend to be) caring and compassionate for a little while, but they were never truly good inside, so when the flesh urges came back they had no compunction with satisfying them. 

2. They fall for some Michael Pollan-esque line about humane meat (probably because they are desperate to eat meat for selfish reasons, so this is just an extension of 1).

3. They cannot be both healthy and vegan. This is a rare and bizarre scenario, since even someone allergic to vegetables can be a healthy vegan. But if it does really happen, these ex-vegans don’t count as ex-vegans because they might still be decent people.  

But why do ex-vegans have to be so noisy? Since there is nothing wrong with veganism, it can only come down to guilt, defensiveness or a combination of the two.

Okay, so what do we do about ex-vegans? Do we ignore them? Comment consistently on their blogs? Wear a “Never Trust an Ex-Vegan” T-shirt? Tell them oysters are cool with vegans now and try to convert them back?

I’m probably the wrong person to ask.