What causes stop and go traffic in all lanes on high speed roads?

More specifically, when conditions are perfect for driving, no construction, no stalled cars, no accidents, nothing that would indicate why the slowdown happened.

There’s been so many times where I’ve sat in stop and go traffic just to see no reason why the slowdown should have happened once the speed picks back up.

I’m thinking maybe there’s a crash, maybe there’s construction, or something is wrong. But then upon returning to 60+ mph, there was nothing!

Genuinely curious why this happens