It's reasonable that bikers would be annoyed with someone riding in the middle lane and not moving over
It's reasonable to think that annoyed biker could take that annoyance too far, and do the actions they did.
The actions they take in response to that reasonable annoyance were entirely and unquestionably NOT reasonable
It's like saying its reasonable that someone would be annoyed by a crying baby on a flight...if said annoyed person punches the baby, that punch is unreasonable regardless to the fact their initial annoyance was reasonable.
Reasonably being annoyed by something doesn't justify an unreasonable action in response. It was stated more to essentially present "motive" for what we witnessed by the bikers at the start of the video. Motive = a reason, not an excuse