I think it was a combined, synchronized effort

Bus on left hand side of screen sealed the deal, but it was a concerted effort of fuckery

Each one of them screwed up independently. It takes a concerted effort to reach this situation.

I don’t know what’s going on in this picture, but, whatever it is, being accidental is last on my list of possible explanations

It's a deadlock situation and can arise without any screw ups. Imagine the buses being 5x as long and all reach the intersection at the same time as an example. All have space to enter the roundabout but they'll reach the same situation as in the picture.

To fix this people uses signals and can be done by having the buses follow "Only 1 or 2 articulated (double) buses are allowed into the roundabout at a time". A rule like that might be required if you have a lot of articulated buses. The risk of this happening depends on how many ways you can enter a roundabout no a bus like that, how long the buses are, how frequently they run and how big the roundabout is.

In my opinion the stupid bus that entered the round about when already 3 buses where in there.

I mean get that there was space, but man you could have waited 20 seconds and everything would have been fine.

1 of 2. Either the car drivers going on the inside of the busses, hindering them enough space to make the turns, or the bus drivers allowing it to happen.