The numbness is caused by either pressure on a nerve or lack of circulation due to compression.
I used to lose sensation in my fingers on each hand starting with my pinking and working towards my index fingers as it progressed. It seems to be linked to hiking with a heavy pack without using trekking poles. I never did find out the specific cause, but it was nerve related and had to do with my posture and how I held my arms with the heavy pack on. I never happened using trekking poles, I am guessing because my posture and how I carried and used my arms was different and didn't impact the affected nerves in the same way.
Your sciatic nerve runs from your lower back down through the posterior side of each of your legs. I'd guess that something around your waist or lower back is getting compressed to the point that a bundle of nerves is affected to the point that your leg goes numb. As others have said, a bulk pants belt cold do it. The waist design of the pants you are wearing, even the design of the pack and pack hip belt padding could be doing it. As much as I love my Exo 2K, the waist belt and padding are pretty poor on their packs.
I'd experiment with a few different combos of pants, belts, and packs to see if you can find things that either palliate or provoke the issue. Also try moving the waist belt around on your hips as well, where it rides, higher or lower, may help the problem as well.
If it's a circulation problem, the root cause is likely the same, something is too tight. Circulation issues are usually associated with loss of motor-function and/or forms of pain though, whereas nerve issues are usually numbness or a sharp, shooting type pain.
