You charged that as percentage of activity. Any way you can plot that in terms of a raw metric... like number of lines changed or number of commits.
I'd like to know if the decline in the percentage is actual volunterr falloff or is just showing up because the employed activity has increased significantly. Just graphing the percentage doesn't tell us that. It could be that both are going upwards in terms of the amount of actual work being done, its just employeed activity is rising faster. Subtle but important.