I never use nutrient except in my starters and I get those quite often. I would guess that yeast nutrient isn't the cause, but I'm afraid I can neither confirm nor deny that statement
Have you become less patient? There are a lot of bubbles on the surface in that pic, I would guess that yeast isn't quite finished working yet. I would give it another week for the yeast to clean up after themselves. When the yeast finish with the sugars they will go back and clean up after themselves. It really helps the flavor of your beer to let them do so. After they are done they will fall out and go dormant and the CO2 production will stop. There is still CO2 in solution and some of it may come out and give you bubbles in the airlock, but your beer will look different. The cake will be thicker, the beer will be clearer, and the bubbles on the surface will be all but gone. Then it's time to transfer to the keg/bottles. That's my 2c, but I have found that patience helps produce a better beer.