I think because of low supply to the market   

Many factors were need to be considered for the reason for this. It can be a normal reaction of the market and many were just taking profits. Remember that this season, many traders or investors are making this move so it is normal for a coin to plummet or dip. It will eventually climb to its range once investors come back in. Bitcoin is also volatile regarding news and many were being affected by FUD. Because of this many were panic selling and regret later on. These news, intentional or not, extremely affects Bitcoin which can be an opportunity to buy or the other way around. Just believe in Bitcoin, you are in good hands.

This week every coins are going down, maybe cause the korea problem

A couple of days ago South Korea announced it would ban cryptocurrency exchanges. Right now it's not clear if it'll really happen or not, but it had it's effect on the crypto market already...

I don't think it will go down deeper $10k. The huge dump of bitcoin price really affect the trust of some individual who are still doubted the capability of bitcoin. That's why they are just selling their btc even in lower price in order to recover their investment though they losing which is for me that is not a good idea. I know btc will recover soon, we have to patiently wait for that and then sell.

 Always there are two sides of a coin, always there is a dark of night after the lights of the day.  Up and down is definite and certain in a circle which is always moving.

But some times it is natural and other time it is man made.  Every business idea starts honestly and with hard work but when the time comes getting fruits of it some cheaters; fraudsters ; thieves; decoits will attack and will try to take away the profits from the innocent; honest entrepreneurs, traders, inventors, investors etc. 

I think the same thing happening here in crypto market.  Like in share market there are speculators all the time pushing and pulling the share market upwards and downwards and surprisingly they gain from all the ups and downs of the market which is dancing at their commands, but innocent investors unknown of all the cunningness of those speculators, loss their hard earned money they invested in the share market with the hope of earning some money from there.

In the same way I think it is happening because many speculative investors have entered in bitcoin business and they are all creating this type of speculative artificial ups and downs of prices of bitcoin.  Investors must be careful while buying and selling because it should be done only after deep study of the market and not blindly otherwise he will burn his hands only.

bitcoin is going down because if you check historical data,  this is the period bitcoin dumps first each year and also it probably does do because many people cash out of crypto  at the end of each year and are slow to invest back at the start of a new year.

Dont be afraid because correction is normal in any investment like cryptocureency. This is healthy run of bitcoin so it is not just growing without any correction because if the correction takes a place then will be huge correction and its very dangerous.

Decrease in bitcoin, increase in demand. Just strategy

I think there are some people who keep on hold bitcoin because last month bitcoin price was very high and then at the end its price become low so the reason was people keep it on hold because first they were selling bitcoin when bitcoin price was high at start of the month and then when it’s price start going more high so people stop selling and people keep it on hold so the price become low and same its happening again here.