With regard to a line of people or cars, a noun that could apply to this phenomenon is slack and how line participants react to it and how it seems to accumulate.
slack (n)., take/make up the slack
Cessation in movement or flow
A part of something that hangs loose without strain
A portion (as of labor or resources) that is required but lacking
m-w
That part of a rope, sail, etc., which is not fully strained, or which
hangs loose; a loose part or end. Also figurative, esp. in to take up
the slack, to use up a surplus or make up a deficiency, thereby
maintaining or returning to a stable condition; to hold on the slack,
to skulk; to be lazy (1864 Slang Dict.). OED
It's like a line of traffic starting up, or the tide turning; first it
has to take up the slack, so in the beginning it will seem like
nothing is happening. Terry Bisson; In the Upper Room and Other
Likely Stories
The two cars in front of me crept forward to take up the slack.
R. T. Anthony; Mountain of my Dreams: The Middle Years
At every gap in the traffic, I shot out to my maximum seventy, and by
the time he made up the slack, there was more traffic. He was
trapped. Robert Morton; Just a Little Watermelon Talk
Danny let the car idle for a moment, then began easing it forward,
taking up the slack between him and his new prey. J. Gelb and M. Garret; Hotter Blood
Driving habits in response to high traffic volumes stretch the slack
out of the system. Drivers ride on the edge, saving a few seconds or
minutes a day, but substantially increasing the risk of much longer
delay on some days. T. Moore et al.; The Transportation/land Use
Connection
Found thanks to Greybeard's stop-and-go wave:
A number of factors determine how it is distributed between vehicles
and how much slack is available as free space. A. M. Okun; "The
Game Plan of Stop and Go"
Note that slack was already in use with reference to the cars making up a train:
The question of how to handle the slack between the cars has been
given special prominence by a paper by ... Railway Locomotive and
Cars, Vol. 32 (1898)