From what I have seen, it more stems from the activism vegans are engaged in more than the actual veganism.

I think there’s nothing wrong with explaining your ideas and why you believe them to those willing to listen, but I can see why pushy activism for any cause can get annoying quickly. There are often Jehovah’s witnesses outside my local supermarket, for example, but they only give you a pamphlet if you specifically approach them

It’s not just pushy, it’s judgemental.

Oh, you eat meat, murderer? Your shoes are made from the skins of defenseless creatures. The sugar you’re so callously adding to your coffee was processed with ground-up bones, you unredeemable monster.

Even the arguments for veganism that aren’t built on animal cruelty still take on an air of moral superiority. Don’t you care about the planet and future generations? How dare you trade carbon emissions for the temporary comfort of a bacon cheeseburger!

The vegan movement has always been associated with anger and vitriol, even if that anger is justified.

In my experience, your first sentence sums it up nicely.

They assume a moral high ground because they’ve adopted a diet that is generally deemed healthier and better for the environment (I don’t always agree with this).

But unless they’re also doing all the things we could all do better (e.g. not buying new, not upgrading the the latest and greatest, not taking 40 minute showers, not eating out every second day), they’re only somewhat less guilty of environmental damage than the average person, they’re taking a generally undeserved “holier than thou” position and then shoving it down your throat. This isn’t everyone, and I don’t really care what you eat, but these are the vegans that get under my skin.