That's definitely a hypnic jerk; they occasionally happen when you're in a half-asleep state and are on the verge of falling unconscious, and are often accompanied by a very quick falling and hitting-the-ground sensation. Mine often involve a trip of some sort, too, but it's wierd because they happen so fast!

Lack of sleep, overtiredness, and an improper sleep schedule can contribute to hypnic jerks, but everyone experiences them once in a while.