I've found it to be not uncommon among heavy smokers. I can only give you my hypothesis based on my own little subjective study of this phenomenon.
The same thing you described happened to me for the first time about a year ago. I'm pretty sure it's not related to the fact that the smoke itself is depriving the brain of o2 because I don't smoke my weed, I vaporize it.
The first time it occurred - not coincidentally - happened around the same time I realized there were much greater benefits to holding the vapor in my lungs than there was for smoke. From my own experience as well as a number of reputable studies that were done, it's been shown that there is very little - if any - benefit to holding your breath for more than a couple of seconds after taking a hit of smoke. I had been assuming the same was true of the vaporizor, but I didn't find the same diminishing returns from holding the hit in my lungs for a longer period of time.
It almost always would happen when I went from leaning back (for more than 15 minutes or so) in my chair after vaporizing a good bit of the sticky icky. It would usually happen immediately after several large hits and/or while I was holding a large hit in my lungs. I often would be on an empty stomach and I can always feel it coming on for about 3-6 seconds before it occurs.
I would be aware I was about to black out so i'd try to brace myself, then I would immediately come-to on the floor with whatever I tried to grab on the way down on the floor with me. It would take a few second for everything to become coherent and then i'd be fine (often with a few scrapes and bruises).
I started doing it on purpose in a controlled environment to see if I could figure out how to control it when I felt it coming on. The answer is: yes. If you can find a place to sit or lie down, the feeling will almost always pass. If it has to be the floor, so be it.
If you're standing up, no matter how strong your will, when this happens you are going to end up on the floor no matter what you grab.
My wife saw this whole process take place and basically, it looked like I was having a seizure while standing up (trying to keep my blalance). But I lost motor control eventually and just buckled, then jumped right back up as if everything was okay (as I tried to make sense of what happened).
So my hypothesis is this:
obviously the blackout happens because the brain is being deprived of oxygen when I stand quickly from a reclining position while holding my breath (or perhaps out of breath to some extent). The contribution of the marijuana is not so much in that the smoke is depriving you of needed o2, but the process of smoking itself is.
The headrushes would be normal for many people with low or borderline low blood-pressure but I think they're clearly compounded by the reaction of thc or some other cannaboid with your brain. Obviously I have no clue what that may be.
What I learned from my ill conceived autoexperimentation is that under the right circumstances I (and maybe you) can induce a very brief seizure like state that could best be described as 5 seconds of what it would be like to simultaneously inhale ketamine and nitrous oxide.
More imporantly, I learned that these headrushes - while enjoyable on a boring day - are also very avoidable.