Cyclical output counts from a D Flip Flop, what is this effect called?
The effect is basically Aliasing. 

Your clock makes a digital sampling system, whenever the data signal is less than half the clock your data signal will determine the number of edges in a window large enough to count them, when the data signal is larger than half the clock frequency they start to interfere in a pattern that is, when taken with sufficient samples, a triangle waveform in the way you plot the data.

This is not at all strange, and fully predictable.

When the clock and data are exactly the same frequency, the Data will always have the same state at the rising edge of the clock, so no transitions will appear at the output. When the data frequency is half the clock frequency all transitions in the data will appear on the output, because the clock can "accurately sample" the data.

Then as you go lower, towards 0, you get less and less, not because of interference, but because the data signal just decreases in frequency. It still gets to the output just fine, because the clock can "easily sample" the data.

The reason the pattern is repeated, is because it is a purely digital system, so at each clock transition the data is sampled, and what ever is seen on the input is set at the output. If the data transitions exactly twice per one clock transition (double the frequency) the sample point will still always see the same data level at the rising clock edge, so again no output transitions are present. But at exactly three data transitions per clock, the data will again exactly toggle between each rising clock edge, again giving a maximum.

Etc etc etc.

So, aliasing.