What is it that makes waves travel in sets?
-- Forever in your debt, Hillcrest
Imagine Grandma Alice trying to get the elves ready for a walk. Grandma's barking orders, terrified elves in chaos-- boots, coats, Ray Bans flying. Gradually the group gets organized and heads out the door. The first few minutes, elves are stepping on each other's heels, a few elbowing past the slowpokes until they finally string out in predictable groups-- fit, energetic elves surging into the lead, average Joes in the middle group, grumpy couch potatoes lagging. Substitute a wave-generating storm for Grandma, wind waves for the elves, and deep-water wave sets for the final result.
Wind energy is transmitted to the surface water, which creates chop traveling in the same direction as the wind. Choppy waves consolidate their energy into rolling balls of under-surface energy (swells). The more energy the swell has absorbed, the bigger it gets, the faster it travels. Given enough distance, the faster swells overtake the slower, and things settle out into same-speed wave sets. There can be as few as three or as many as a dozen or more waves in a set. Actually, wave mechanics is a lot more complicated than this, but we're talking Grandma and the elves here, so consider yourself lucky to get this much.