I'll reply to myself to avoid editing the above. The other user made me realise that what I said about pass-aggro is unclear - since the expression is used with multiple meanings.

In this specific context, by passive aggressiveness, I mean "an utterance showing politeness as a disguise for rudeness".

I'll give you guys an example. Imagine that Alice says "I saw a potato tree today". And Bob replies to Alice one of the following:

1. "Potatoes do not grow on trees."
2. "Potato tree? Are you braindead or what?"
3. "Yeah sure, and I saw some unicorns today. Because you know, potatoes totally grow on trees, right?"
4. "Oh dear perhaps you're a bit confused, so let me enlighten you. Potatoes do not grow on trees. I understand that this might be a bit too complex for you to understand, but put on some effort, okay?"

The first three are not pass-aggro. #1 is simply dry (no attempt at politeness or rudeness); #2 is simply rude (I'm typically OK with that within limits). #3 uses irony and sarcasm in such an obvious way that it comes off as simply rude, there's no attempt to use the irony to disguise the insult. Only #4 is pass-aggro, as it calls Alice stupid and lazy in a disguised way.

I tend to block people who do this because they rub me off the wrong way - it shows a lack of dignity to be upfront that you don't see in plain rudeness.