Instead of moving over well before they know they'll need to, people just try to get as far ahead as they can and cut their way into the next lane. The idiot moving at the last second will have to slow down to avoid missing their exit or getting shoved onto the shoulder/median, and then they'll be trying to cut into a lane where the traffic is now moving faster than they are. This results in "brakedancing" as someone will eventually have to slow down to avoid hitting this person, and everyone behind them will follow suit. It's bad enough when just a couple of clowns do this and annoy a handful of other drivers, but repeat a few dozen times and - you guessed it - traffic starts backing up altogether. The absolute worst, though, are the arrogant morons who don't want to sit in the line of traffic that's slowly moving onto an exit ramp or the like, so they speed up along the line of cars (either in an adjacent lane or on the shoulder) and then try to cut their way into the front of the line. People who do that make the traffic backups even worse, and should be fined heavily for that self-serving sort of nonsense.

When you know you need to make an exit, or that your lane is going to end or be closed, please do everyone on the road a favor and move over reasonably early. Don't be the ass who stays in the current lane as long as possible just to get a few cars ahead, because even though you might shave a few seconds off your commute, chances are you're going to slow things down for everyone else. That just ain't right!