definitely agitated. its hard to tell without context and knowing the cat, but that tail thing is something they do when theyre amped up. sometimes my cats do it when theyre anxious, or about to have zoomies, or overstimulated from petting. could very well be annoyed but id guess anxious here if he felt like he had to run away from something. probably best to leave him be to calm down. he'll come back around when hes feeling up to interacting again.

Like others say: tail flapping indicates something bothered him. But (assuming he's looking at you holding the camera when he briefly looks up) you're not the reason. He checks you out and seems to conclude you're alright. The way he positions himself also indicates you're not a threat but wants to keep his eyes on you regardless (or the scene behind you). I might be wrong, I'm not a licensed cat psychologist.

In most cases that’s the cat’s “BACK OFF, MAN” move. Further approaching might get you a bite or a big scratch.

As to why; unknown. Maybe scared or injured or just really annoyed. Either way, as someone else mentioned, might be good to let them relax and wind down a bit.

Yes this cat is pissed off, give them some space to calm down. They will come back when they're comfortable again. Cats do a lot of communication with body language (as well as scent, but our noses and brains can't process that very well). Try to learn the body language of your cat to better understand what it needs or doesn't need. They are very good at communicating, but unfortunately humans don't always understand all of it.

how well does that cat know you? could just be scared about meeting someone new

Giving mixed signals to me. Tail wag suggests he's nervous; but then he laid in a way that exposes his belly which signals he's comfortable. 🤷‍♂️

Yeah that tail wag looks annoyed to me. Looks like the cat is trying to remain calm and patient despite how he feels so he’s separating himself from everyone.

The only thing missing is the licking of the lips. I’ve noticed a few cats and dogs do that when they are uncomfortable and I feel like this cat may have done that too. It’s like a gulp and licking of the nose a few times, like 2-3 times. Could be breed dependent, though, but likely happened before this video was taken.

The move from laying on his belly to starting to lay on his side is a good sign he’s not too annoyed/angered and is calming down. Bellies are very vulnerable positions for both cats and dogs so for them to start to show it is a good sign they’re in a good mood/comfortable and not on edge. It’s a very slight change I see near the end of the video.

This could be because you followed him. He wanted space and he feels violated, for lack of a better term, because you’re following him when he wants to be alone.

I’d say just give him space and let him come back when he feels up to socializing again.