It is a documented phenominon that a group of waves travelling through a medium will re arrange their energy distribution (amplitude/size) to have smaller waves at the front and back of the group, with the biggest waves in the middle.
So a group of waves, just for this case let's say five, all started at the same point with regular frequency and amplitude, will gradually change from a group of five identical waves, into a group that goes smallest,middle, biggest, middle, smaller.
There is actually no scientific theory explaining why this happens, but it does. One theory to do with it being the most effecient way to move a group of waves along sort of explains how, but that doesn't explain why.
The above phenominon combined with interference(from the same source, and secondary sources) ,diffraction and refraction (an effect from waves entering water with a different depth) will probably combine to give you sets. probably.