What would cause this???

I have many pics ot those. One roof, it was caused by lack of cross bracing between the rafters, from front to rear.
All the others? Can’t verify cause and effect, but the majority were missing their 4th. nail on one end. I lifted the tabs to look.

Notice the cracks are at 3’ intervals. My own positively non technical opinion has to do with fiberglass. It’s soft as silk when in it’s woven state. Once it’s been embedded in anything, it is brittle. I’ve worked in bodyshops and built boats, so I know this for a fact.
And, you can’t get splinters when in it’s soft state. Shingles throw splinters like quils.

I think the slight movement allowed by the one un-nailed end ‘could’ be enough to cause splitting.

I have no scientific data, just observations from experience.