Around my way they have put Shell Grip on the roads just before traffic light to improve skid resistance (which it did when it was new) but now the surface is very worn and its noticeable that the tarmac underneath is even more worn with huge potholes and other faults. This damage is restricted to the areas covered by the Shell Grip though (the rest of the roads haven’t been resurfaced).
Does anyone know why this might be happening?

Perhaps the increased grip of the road covering provides a greater sheer force that is transferred through to the underlying tarmac thus increasing the risk of potholes?, dunno – could be entirely wrong but it’s the same on a few corners up here where it is laid down. The surface is all rutted and quite often potholed on the most common route through the corner, very similar to stutter braking bumps you see at trail centres on the way into berms.

Or perhaps it’s just been laid in a rush on a wet friday afternoon

Two main reasons this would occur.
The first, as SF said, is that the increased shear forces transferred to the pavement surface by the antiskid have eventually overcome the adhesion between the binder and the aggregate, leading to loss of coarse aggregate (fretting/ravelling) of the surface course.

If the binding material of the antiskid was laid too hot or went off too quick, it may have caused hardening of the bitumen in the pavement resulting in less adhesion and durability.

Either way, nasty potholes grow.