This is an idea...

I just started study medicin here in Sweden and I came across a case were the patient had short "blackouts" were he felt gone for a while and then came back, as if he fell asleep a short while. I dont know if he ever had any neurological trauma or deficiensy. But after a LOT of testing, they came to the conclusion that something was wrong with his vestibular system. He also had high blood-pressure btw which might have interacted with this problem.
The measurements showed that the bloodsupply to his brain almost stopped for a while which caused him these blackouts. Nothing dangerous, except for what injuries can be caused by getting a blackout while walking, driving etc.

I cant give you an answer other than it sounds like what you experienced. An injured brain also normally have a less accurate vestibular-control and also yhe brain itself is more sensitive to changes in bloodpressure, o2 supply etc!

This is possible explination.

Did you sit in a static way for a long time before this happened?

My braininjury has sometimes caused me to feel dizzyness just like a low bloodpressure in situations were I have been in a static position with my head/neck or if I get up after sitting down etc.

Emil