There's no sure way to identify the source of cracks of this sort once the wood has been converted into boards and especially when already placed, so you're only able to see one face.
I’ve encountered this in the past when we built log furniture, and someone told me it’s caused when the tree is felled, like if it takes a bad bounce.
That type of stress crack is a possible source, but because of the complex 3D nature of cracks in wood (they may follow grain boundaries only very approximately!) what we're seeing here could even be cross-sections of drying cracks that angled across the wood rather than strictly parting the grain as we usually expect. Without information about where the board was in the original log and how it was sawn 9which sawing method to convert from log to boards) there's no additional information to go on.
it starts by figuring out how to describe it
Just "crack" or "cross-grain crack" are going to have to suffice for describing this verbally. A picture is worth a thousand words if you need to show the exact nature of the defect to anyone :-)
But I need to solve this problem
I'm afraid there's no good solution here, particularly if you're seeking a seamless repair that can't easily be spotted with the naked eye.
It may sound trite but the 'fix' for this is not to use the board in the first place. Or to saw off the affected portion if that wouldn't leave offcuts that are too short to be usable.
Cracks that go across the grain like this, no matter their cause, can't be filled in a way that makes them invisible — although fills that cross grain lines can sometimes be done in such a way that they look very good from a specific angle they always fail scrutiny when looked at in varying lighting conditions from some other angle (or possibly most other angles) because of the way that the surface and structure of wood refracts and reflects light.
Regardless of this, where you do want to go ahead and use the wood, and fill the crack, the time to do so is when the wood is still bare. Post finish application you always run the risk of the finish absorbed by the wood fibres and of course whatever has wicked into the crack itself, interfering with the bond of the filler material. This can cause the filler not to stick adequately to begin with, or to fail prematurely in service.