It's evolutionary. Turtles who moved to the sea were less likely to be eaten by birds. And the the faster they raced to the sea, the better the chance to survive and pass along their genes. Multiply that over millions of generations and an instinct is created. Ultimately that's what all instincts are, baby creatures born with the instinct that benefits their survival were more likely to pass on their genes and make more babies. It takes a long damn time to do, but evolution is slow.