SP can happen when you fall asleep aswell as when you wake up. When you're feeling dozey, you're about to fall asleep and end up in the "inbetween" state instead. It's even more common to happen during naps than during the night or before bedtime (as you're usually so tired at night that you lose consciousness very fast.)

Why it started when you were 26 I can not say. Unless you started meditating or focusing on your awareness as you went to sleep, I would think it just happened by random. Although why it started from there might be because of the fact that you're afraid you will have them. When you lie there trying to sleep, and fear you will have one. You keep yourself awake just like anyone trying to WILD would do.

This applies when having a deep fear of having nightmares, especially a re-occuring one. You fear it will appear again and you end up actually inducing it by thinking of it as you go to sleep.

Perhaps trying some meditating when falling asleep will work, and if you want to lose consciousness as fast as possible then there's only one thing to do, work out until you're dead tired!