Market has been sinking for a long time... although the news comming are mostly good.. and today a drop fo 12% of marketcap without a clear reason?¿?
i only can think about : market manipulation!
I think you're jumping too early to a conclusion. This might a thinking fallacy (ignorance fallacy). If you don't know, doesn't mean the answer is the only thing that you can think about. Maybe they know what you don't know

BTC is supposed to be decentralized.. but i heard some time ago, this just the other way arround.. 2or 3 big mining groups have a 90% of the supply...
Obviously, there is no 2 or 3 party that control the 90% supply of Bitcoin. Did you mean 90% of the hashrate? That's not true. Even the cumulative top 5 total hashrate "only" accounts for 60% of the hashrate.
I agree with the poster above, regardless what caused this drop, right now is a very good time to buy nevertheless. I've been expecting Bitcoin to drop below $6K since a few months ago (read my post history), thought it would never happen in 2018 considering how stable Bitcoin these days but it finally broke. I will definitely buy more of these cheap coins once I got some money in my hands.