As i've never seen any of those people in my entire life, I'm pretty sure I'll be able to look at this situation from a neutral standpoint, so let's analyze it.
At the beginning of the video we can see the Tech and the Chanter standing around and not targeting each other, while the Ranger is invisible. This seems slightly suspicious, but let's call it a tactical move.
At 00:23, the Tech joins the fight between the Chanter and the Ranger after both used their Remove Shock skills and starts attacking the obviously squishier Ranger. Looks fine.
00:30, the Ranger dies while the Chanter is at 25% HP and blinded by the Sandstorm Trap. The AT - instead of hunting him down - makes a few steps in the opposite direction and then runs after the Relic. Doesn't make much sense, but the round was literally about to end, so yea, whatever. But the fact that the Chanter didn't target the Tech while running away is still a little bit weird... As if he already knew the AT wouldn't attack him.
01:32, the Tech could've done the last hit on the Chanter, but didn't.
02:35, again Chanter vs Ranger. Both are at 5% HP and the Tech incs. The Tech decides to go after the Ranger which is behind a corner and ignores the Chanter which is right in front of him. He kills the Ranger and double boosts away from the Chanter instead of attacking him. This makes absolutely no sense. He could have safely killed both of them if he attacked the Chanter first and the Ranger after that. Not attacking the class that can heal itself while it's at 5% first and giving it actually time to heal itself is kinda sus. Completely ignoring the class is even more sus.
03:10 the AT takes fall damage... For some reason. AT and Chanter are both in each others attack range, but instead of attacking, the AT targets the Ranger (03:14) and then removes his target completely. The Ranger jumps down and gets completely raped by both of them. The Tech loses 30% of his HP and uses Remove Shock in the process. The Chanter, instead of attacking the AT, which is in the first place and an easy target at this moment, ignores him and runs away in another direction. The AT doesn't bother to select the Chanter, because he seems to know that he isn't going to be chased anyway.
03:50, contrary to Pacts' statement, the AT does not actually prioritise the Chanter. In fact, we can see him use Cinder Cannon and Gattling Gun (the ranged skills) on the Ranger, followed by Chilling Wave and Riplash, which are both frontal AoEs, immediately followed by Electric Shock on the Ranger. The chance of the AT having targeted the Chanter at any time is very close to zero.
04:11, the Chanter has already respawned, but completely ignores the 30% HP Tech which is standing right in his face. He attacks the full HP Ranger instead (04:24).
04:48, the Tech does the last hit on the Ranger and yet again completely ignores the 30% HP Chanter which already used Remove Shock. None of them is targeting the other one.
05:08, the Chanter is below 20% HP and rooted by the Ranger. The AT could easily kill him by using Cinder Cannon. The AT decides to attack the 95% HP Ranger. After they both kill him, the AT just sprints away.
05:30, the AT sees an easy target and runs towards the Ranger. He buffs up to the fullest; the Ranger dies in the water or whatever that shit is and the AT just does a 180 and runs away. Fully buffed. Ye.
05:55, the Ranger is the last one in the ranking. If the Chanter joins him in the fight against the Tech (which is very close to him, at 60% HP, rooted and without Remove Shock), he would win. The Chanter instead decides to kill the Ranger and to run away from the Tech.
To summarize it: The Tech has hit the Chanter, like, twice? With his AoE, while focussing the Ranger. So basically by accident. The Chanter on the other hand didn't hit the AT even once.
Btw, I don't think the Ranger "stopped trying" or "started trolling and fucking around" at the end. Maybe he was a little frustrated and did some mistakes because of that, but I'm pretty sure that it wasn't on purpose. I mean, at 00:05, he uses Bestial Fury and Focused Shots for no reason, losing his Hide and the advantage in the process, while standing right beside the Chanter. That's just how he plays.
He also could have milked it much more while he was standing on the balcony at 03:15, to make the teaming even more obvious, but he didn't, since he was aiming for the win and not for some unrightful ban or whatever.
Also, the fact that the Chanter and AT are seemingly both from the same legion and the legion seems to have some beef with the Ranger doesn't make it any better.
My opinion: 100% teaming, 100% against the rules, 100% deserved a temporary ban.
It's fine to join the arena with your buddys, but damn, act accordingly.