It has nothing to do with aircraft, and everything to do with random chance. Basically, it is more likely for time-series data to have clumps than to be perfectly even.
Imagine you throw a three darts a board. Which is more likely: all darts are equidistant, or 2 of the darts are closer to each other than the 3rd? Now imagine it's 3 events in time rather than 3 darts on a board. And the same logic applies to an arbitrary number of events, of course.
