Short answer: high levels of anthocyanins.

Why it has this is another matter; one possibility is a garden escape of a selected purple-leaf cultivar breeding into the local population.

Resin

Some kind of stress I think.

A brief note:

Recently doing a project to root 30 cuttings of Lamium maculatum 'Beacon Silver'. Most of these rooted in about 7-10 days in water with large growth of adventitious roots from the stem internodes and a few roots from the nodal areas. 4 of the 30 did not root in that time frame. The leaves, usually a light green edge and a silvery center, turned a purple color like these plants. They rooted in about 17-20 days and most roots were from the nodal points. After growing in potting soil for 2 weeks, all the purple color had disappeared from these plants.

Not sure what this implies, or how it relates to your question, but it seemed to show an interplay of this coloration with a low auxin condition.

George