My verdict: He wanted to return the favor to the tanker driver. Notice how he activates the right indicator immediately the tanker overtakes him (min 15:46), then tries to get into the opposite lane, readying to overtake the tanker, but notices an oncoming lorry, which forces him back to his lane. He attempts to overtake again (16:06) but returns upon seeing the canter. At this point, I am thinking he over-steers to the left, then tries to correct, but goes too far to the right, by which time the canter is too close.
So it looks like anger, inexperienced driving, fast acceleration, possible modified suspension, incorrect tire pressure, a pompous tanker driver, and a canter driver who dislikes dirt, all did him in.