Questions about an experience tonight

I have some questions about something that happened to me tonight.

After waking from a non-lucid dream, close to 5 am, I decide not to move, to see if I go into a lucid dream. I try not to focus on it or throw away other thoughts. I simply let my mind think what it wants.
After a few seconds, i start to feel vibrations and my heart beating faster. I think something will happen but i try not to focus too much on it. What happens, happens.
Gradually, i see an image forming. I see the top of a tree, green leaves move with the wind, vivid colors ... I realize that i'm dreaming.
I have not concentrated on practicing some special technique. I must have made one accidentally. What was this technique i've done?

Another thing is about what happened to me in this dream.
I fell asleep lying on my back and my head to the side. On the dream I watch the tree, in the same position i had fallen asleep. I'm hovering beside the tree. I can not move. I know my body is out of the physical body. I focus on moving only in the dream but I can not.
Suddenly, I start to get away from the tree in the same position and i pass through the scenario. I go through a field with trees, then some land, then some warehouses, kinda like an industrial area. After a few seconds I wake up.
Why couldn't i move in the dream? Why was something pulling me away without my control? Could i have done something?

Sounds like a DEILD, a dream exit induced lucid dream. When you wake up from a dream you normally just lay still and fall back to sleep directly into a dream, but if you notice the awakening and lay still, you will go right into a dream and can retain awareness. It is basically a short cut for WILDing.

The reason you couldn't move and the dream was taking you away is not because the technique, but because of your dream control. When in a normal dream things like this happen all the time, but you think nothing of it because you are not lucid. When something like that happens when lucid you notice it more. It doesn't have to be that way, you simply need to practice some control. I probably would have teleported away or imagined some chains around me to break and then imagined a monster pulling me away, then I would break the chains and kill the monster. There is innumerable amount of ways to break it, but the most effective would be to just known that you are on 100% control and that it is all just a dream and knowing what that means.

The number one thing to do now is to not worry about it. :P if you worry, then all your lucid dreams could start like that. Just think of it as a fluke and move on, if it happens again in a dream then defeat it.

Hope this helps. :P good luck.

Thank you, BrandonBoss. Very helpful.
I have another question though, about the vibrations. Why did i feel the vibrations and heart beating faster? Was it because i entered directly from the waking state do the dream state? If so, why come we don't feel this when this happens, for example, when we take a nap or are too tired during the day, we close our eyes and are dreaming straight away?

Funny thing about vibrations. I'm sure we have been getting them all our lives, but simply didn't notice them, because we were not paying attention. I have gone up to the restroom thousands of times, never felt them upon returning to bed, but started noticing them since I started lucid dreaming and learned about them.

I believe, that vibrations is when our subtle (dream, astral) body is raising it's vibrational frequency in preparation to detach (or better said to access another energy plain/dimention) from our physical body. I also believe, that this is happening every night, with subtle body serving as an interface between physical body and subtle energy. Our physical body is recharging every night like this. At least, this is what I think.

But for the purpose of lucid dreaming, when I feel the vibrations, I can just get up or roll out of my body into LD/OBE.