Similar deformation in two tires (1 year apart)

What can cause this deformation in the tire tread?

I had this happen to me late last year.

On a different tire (but the same batch), I had a very similar issue the year before with a similar deformation. Date code is week50, 2018. Both were on the rear axle of our 2010 VW Routan minivan.

Would age cause this? Or some other issue? Manufacturing defect?