I have had 'sort of' similar experiences.
This only happens when I'm outside, with a crowd of many people, or with loud sounds (particularly people's voices) coming to me from many directions. I begin to feel 'out of my own ego, escaping it but still there, yet enough to be aware of myself...'
And it just feels unbelievably strange... so strange that words can never explain it, because it's just something phenomenal... And I become afraid of these sounds, as if they're not real, they sound warped, or from a vortex, or something

I feel as though I will never survive... almost at the presence of death, yet nothing happens; I possesses a fear that is totally an illusion. I fear of everything around me. And everything seems 'fake', as you mentioned earlier.
And the instant I return home, 10-15 minutes later I slowly 'get my ego back together', and feel what I did once before.
Odd, this. I read somewhere the brain can become imbalanced/a lack of particular chemicals, such as serotonin that cause these sort of effects.
You enter an area, such as the street, a pathway that 'triggers' something in your head to create this sort of 'reaction', thus you gain that feeling/unbelievably strange perception/whatever you want to call it. You have mentioned yours lasts a couple of seconds, what do you mean by that?
Do you mean, a second and then nevermore? Or does it go on and off, on and off, fades, comes back, then finally goes away?
Mine could be described as a 'neverending' feeling that does end, but in a period of a few hours. During this time frame, I feel out of myself, just crazy, and experience 'deja-vu' type feelings along the way that seem to come back and fade in a cyclic manner.
It is important to note that I only feel like this once I think of a particular thought - I can't pinpoint it exactly, but I'm sure it's one thought - the leads into this 'phase' I have. The thought is like a path with one dimension, you can't follow any other thought processes once you've thought of it because this one is taking over everything else... and once I feel like that I regret I thought that thought.
And I only think like this when I'm around people... it's a paradox almost.
edit: test yourself if you've felt like this by yourself at home, doing an activity you do usually every day. Do you still get the feeling sometimes? If you're into what you're doing, do you ever get it?
If you never do, it could be hypothetically said that you only experience this feeling due to a situation you think differently in - and hence this causes the chemical reactions in your head to react in a different way...
NOTE: I am not here to give a theory lesson. This is only what I believe, my observations...