Jumping sharks!

Yesterday evening Tijn and I went out for a sail. The idea was that, since heavy winds were expected, we should take advantage of them. The wind was a flukey 5-9kts from the Southeast, which was interesting in itself, because the swell was the usual Southwest, so you could really get going straight uphill. We just sailed back and forth, mostly off East Beach. The notable events of the evening were 6 or 7 sightings of jumping sharks. These guys were maybe 4 feet long and sort of thin, and they were jumping straight up out of the water. They would clear it by about a foot and fall back in. It was really beautiful and very mysterious. What would make a shark do that?