Andrejs:
Here no so much drivers shortgate,just big drivers turnovel.To many drivers jump from agency to agency without really good reason.
I think I’ve just touched on this in the “driver surveillance” thread.
It’s all too easy to get banned from site for backtalking some client firm management, so the driver finds themselves having to go to another agency to get the regular work somewhere else.
Agencies tend to have these major clients, which once you get banned from site at - make your continuing with said agency rather difficult…
Basically,
(1) Client firm treats driver like dirt, instead of a professional on call to do the job. Argument ensues, usually about some H&S aspect the driver is not prepared to breach (as if there were a Union there!) but alas, driver gets short shrift, and gets banned from site for simply “arguing with the gaffer when you’re only agency scum nobody”. I’ve been banned from sites over the years for things like “Refusing to use a visibly damaged, and unroadworthy vehicle” and “insisting I got paid for a shift I turned up for, that they didn’t want to pay for”. 
(2) Agency doesn’t want to let you go (as prompted to by client!) because you’re actually more needed by the agency than they let on. They put you into another client instead.
If there is no such client available, then comes the time to move on to another agency with a fresh set of clientel on it’s books.
Thus, if a driver moves on seemingly “without good reason” - it’s down to the reason “The agency didn’t have a big enough book of clients to keep that driver in regular work once they’d been banned from site somewhere…”