Why do I lose my train of thought if I turn my head at night when I'm trying to fall asleep?

You left rem sleep and your parasympathetic nervous system kicked in

Is this why you call yourself “Flip?” lol
I’d surmise your brain rolls over and flat lines what the other side was just thinking about. You know, like a big ball of bread dough folding in on itself.

Change in brain waves. When you turn you probably are also change your level of wakefulness. If you are also opening your eyes when they had been closed when you turn that affects the change also.

I don’t think that the physical act as such is the cause for such a slip in concentration. It’s probably a second symptom related to the same root cause.

I believe we all have certain set conditions and positions for immediate sleep. I use to fall asleep quickly if I slept on my right side but I found the position was causing pain on my right knee and shoulder which caused me to keep waking. So I switch positions with my husband and now I fall asleep quickly on my left. It took me awhile to get use to it but whenever I just lay on my left side, I fall asleep quickly. So you probably have trained your body and mind to relax as soon as you get into your sleep ready position.