The usual cause is an image, advert or font arriving after the page has laid out, pushing everything below it down. Google wants the score under 0.1; a well built page scores 0.
The fix is reserving space: every image with explicit width and height, fonts that swap without changing metrics, and no content injected above what is already visible. It is a build discipline rather than an optimisation.
A layout shift is also the single most common cause of a mis-tap on a phone, which makes it a conversion problem as well as a ranking one.