Well nothing from a clash could have caused this:

No edges of the field intersect on this area. If might be an over polished area on the die. In the last image you can see the weakening of the dome next to that area. The die may have been polished to remove an imperfection/damage in that area on the reverse die. If you check your coins you will probably find the same markers on each coin if they were all from the same die pair. If you find different markers there may have been more dies affected?