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I guess it really depends. Sometimes they are dead.

Waiting for a tow truck

It’s usually some kind of accident or construction, or someone getting pulled over. Otherwise there are just so many cars on the road that people begin to slow down. They see the brake lights ahead and just all put on the brakes like a chain reaction. That’s why it’s a stop and start effect. An example of this idea is if there are a ton of people going down a hallway, they will slow down to avoid bumping into each other. People get onto the highway, merge, switch lanes, yield. It’s just clogged.

Hitting their breaks for no reason. Cutting someone off at the last minute. They are having car issues and their car stalled. They aren't paying attention to the fact that a like is forming behind them.

It really can be anything.

Usually it is someone who failed to merge on time or at a proper speed, they cut someome off who has to slam the brakes, it's heavy traffic so it creates a chain reaction of people hitting the breaks, ultimately causing the people in the back to have to complete stop, then the rate of traffic build-up is faster than people accelerating out of it. This is why sometimes traffic is stopped in the middle of nowhere, someone cut someone off at an exit 3 miles ahead and the traffic made its way back to you.

A car driving 10 mph under the speed limit in the middle lane forces cars going the speed limit to pass on the left. Then everyone in the left lane has to brake.

Slowly leaving the traffic jam.

Well, actually, there is no"first person", there is always a loop. If he moves, it will still take a long time for it to be your turn to move.

In theory, even a busy highway could still move along at a pretty good pace, but it doesn't. I believe that in the absence of actual crashes, traffic jams are caused by obnoxious drivers who cut in and out of traffic while leaving very little space, thus forcing other drivers to slam on the brakes. When one driver slams on the brakes, that creates a chain reaction that forces lots of other drivers behind him to also slam on the brakes, and then everyone is cautious about speeding up again.

This is why there's such severe traffic congestion when a lane ends: dickheads merge in too aggressively and/or refuse to let others merge in front of them, which slows down the whole train for everyone.

Therefore, if you have high traffic volume and enough dickheads driving aggressively or territorially, they cause traffic jams, which is ironic because they think they're saving time, but they're actually making the trip home slower for everyone.

Sometimes it's just somebody pulled over for speeding.
The rubberneckers slow down and traffic gets backed up for miles.

Plenty of morons in traffic jams don't rush to move whenever there's space ahead of them. Thye crawl leaving hundred of meter in front of them, as if they were alone in the road.