My guess is intergalactic warfare. They can keep their magnetars, my explanation adds intrigue!
God, The Universe & Everything Else
Now, if I knew enough physics to evaluate this crazy guess, then I would, but I won't until I make more progress with my startup!
As for Simons who pays for Quanta, make money first, do theoretical, cosmological physics later!
Also, curvature is a non-local phenomenon. In the sense that if you take a finite volume of space, re-arrange the spacetime however you like, you still have to have the same total curvature inside the volume. You don't have a free hand to reduce total curvature in a volume of spacetime without also altering the boundary.
The cause of these magnetars could be intergalactic warfare!
Who needs nukes when you can blow up a star...