I’ve noticed this as well, first time was Texas in 2020. I’d lived in other places prior and never noticed it. I’ve sinced moved away from Texas, and the bahavior is a lot less common here. I couldn’t figure out why people did it. My theory is, people initially stop at a reasonable distance from the next car and pull out their phone. The next car moves a bit forward for some reason, and the person on their phone doesn’t notice.