I think the story is both more complicated and more simple, and it helps to look at questions and answers separately:
The superimposed curves are very strongly smoothed versions of the data, using a cubic smoothing spline (Matlab's
csaps with parameter p = 0.1).
According to this, with increasing reputation a user has more and more questions, but at some point (about 2000) this increase levels off towards about 10 questions. The smoothed curve is not a particularly good description of the data though. (I'm ignoring the little dip for reputation < 3, which is driven by users with 1 reputation and several questions. Similar for answers.)
For answers vs reputation, the initial increase is similar, but instead of leveling off it becomes a linear relationship in the log-log plot, which implies a power law behavior. The exponent is about 1.11, which means the increase is only slightly faster than linear.
Comparison of the two smooth curves suggests that increase in Reputation is more driven by questions for small reputation, but more driven by answers for high Reputation, with a crossing point at about 370 Reputation and 4.5 both Questions and Answers.
This explains the picture when looking at questions/answers vs reputation. Until a reputation of about 100, the ratio increases slowly up to a value of 1.4, and then decreases faster and faster.
The "third population" speculated about in the comments might appear due to the fact that both the number of Questions and the number of answers vary wildly in the "medium" range of Reputation around 1000. It would be interesting to see whether these relations observed across users also describe the evolution of users over time.
To me this suggests that the number of answers alone is a better match for reputation that the ratio questions/answers, since the smoothed relationship is monotonous starting from a reputation of about 8.5, i.e. across 94% of users.