What causes rim failure where spokes pull it apart?
If there are several spokes showing this failure, it's not because of a single impact.

I think it's safe to say that the rim was simply overloaded and has suffered metal fatigue from stress cycles imposed by the differing forces on the spoke as the  wheel rotates.

Can't say if the failure is because of overloading beyond what the wheel was designed for, or poor design or a manufacturing problem.