The small crater with a piece of stone in it
A pop-out shows up as a small pit, often under an inch across, shaped roughly like a shallow cone. Look closely and you can usually find the culprit still sitting loose in the bottom, a small piece of stone or gravel that used to be part of the finished surface above it.
It is a single, isolated spot, not a crack running across the slab and not a patch of surface flaking off in sheets. That distinction matters, because a pop-out and a crack come from different causes and neither one tells you much about the other.
What is happening under that one spot
Aggregate, the stone and gravel mixed into concrete, is not perfectly uniform. Every load has a few pieces that are more porous or absorbent than the rest. When one of those pieces lands close to the finished surface, it can soak up water sitting in or near the slab, and when that water freezes, it expands inside the stone.
The stone has nowhere to expand into but up, and it pushes a small cone of the concrete above it loose. What is left is the pit you are looking at, sometimes with the offending piece of stone still resting in it, sometimes not.
At this elevation, the freeze and thaw cycle is not an occasional cold snap. It repeats through a whole winter, and that repetition is exactly what drives a pop-out to happen.
Why some slabs show a few and others none
One or two pop-outs on an otherwise sound driveway is not a sign the pour went wrong. It usually means a handful of absorbent particles happened to land near the surface in that spot, which can happen even in a well placed, well finished slab. It is not the same thing as the whole batch being a bad mix.
A pop-out is also not the same as spalling, where the surface flakes off in sheets over a wider area, usually from de-icing salt or repeated freezing across the whole slab rather than one absorbent stone. If what you are looking at covers more ground than a few isolated pits, that is a different question worth asking about separately.
Living with the ones you already have
A pop-out is almost always cosmetic. It does not weaken the slab and it does not spread on its own the way a crack can. Most people leave them alone, and a driveway with a handful of small pits still holds up structurally exactly like one without them.
If you want the pits filled for looks, that is a reasonable ask, but it is not required. Where it is worth a call is if you are seeing a lot of them appear at once, or if the pits are noticeably growing each winter instead of staying put.