I've become convinced that some people just have a stupid competitive streak, that makes them want to 'win the race' on public roads. They lock onto an easily recognizable target as their 'competition', be it a motorcycle, Euro car, or big SUV/pickup, and then when they see it go by them, they figure that they aren't getting where they want to go as quickly as they should. That triggers their overtaking reflex and makes them drive like complete morons. Had that happen to me, this morning, on the 427. An idiot in the fast lane saw me about to overtake him in the second lane, so he sped up until he couldn't see me anymore. Once he slowed down again I passed him which triggered his slotting in behind me, then cutting back into the fast lane to speed off at maybe 40 Kmh faster than he had originally been going.
Then there are the ones who see a motorcycle moving through traffic, somewhat faster than the average, and slot in behind to follow. These people don't seem to understand that 2 metres off the rear wheel, at 100+ Kmh, isn't a reasonable following distance. Had a Subaru do that to me last week. I generally get all the way over to the right lane on the 401 westbound, past the airport, because that lane moves much better. The Subaru driver cut in behind me as I passed, then rode my rear wheel from 427 to Dixie. Unfortunately my camera battery had died by then, due to excessively long hold-ups in traffic. If someone does this to you you can usually get rid of them using the, "How do you boil a frog" technique; slowly reduce your speed until he can't help but notice that he's getting nowhere, fast, at which point he'll bugger off past you.