This is the plants reaction to a stress situation ... you can also see this happening when it gets too cold, in response to chemicals, etc. The plants produce these pigments (anthocyanins). You are just on the edge of tissue damage when this becomes stronly apparent as your image show ... back off the light a little. Often this is a combination to both much higher light levels and temperatures . Remember, just like people, when plants go from one extreme to another, they can shock (example, first day at beach and you over do it.. sunburn). Another thing to remember, with higher light and temperatures comes higher metabolic functions which benefit by corresponding water and feeding.