What causes a sudden jolting awake/startling sensation/adrenalin rush shortly after you've drifted off to sleep? i think they are called sleep starts?

No one knows exactly: These are very common, and called hyponogogic jerks, or sleep starts, and a few other names. They are common and can occur in healthy people. Sometimes they can be due to other problems, like too much caffeine, irregular sleep, and other things. There is a good discussion of it on Wikipedia, See: https://en.wikipedia.org/wiki/Hypnic_jerk