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It's either of them cause the markets is in an unstable rate so when it tends to drop people buy in more coins and when it rises they do same as well, this is because they go for what's good and beneficial to them. It maybe buy time when the price rate drops cause the market seems unpredictable.
The market is never stable and you will always see prices fluctuating. To answer the question from the post you quoted - simply put it is a fluke that is hard to predict.
You may be forced to sell at a certain price - and then the price goes up, but sometimes you buy and the price goes down. These are unpredictable conditions - but you can make a good approach where you don't have to sell everything or don't buy all at once. Sell gradually and buy with DCA - two different situation for good purposes.
Selling gradually is done by those who believe the price will go higher - but buying with DCA is a great approach to make your average purchase price lower than buying all at once. Try to take that approach when buying or selling - at least it can give you peace of mind.
Selling shouldn't be the key factor here but only buying because any investor who is after how he can buy and sell back is a trader and wasting the great opportunity that is presented before him to accumulate and increase his bitcoin portfolio overtime for the future. A new investor or anyone still in his accumulation stage should only keep his bitcoin weekly purchase ongoing, regularly, consistently and persistently for 4-10 years and above till he reaches his bitcoin target.
So that you don't miss out from the compounding value of your bitcoin portfolio overtime. It's when you have accumulated more than enough Bitcoin, that you can start thinking of selling in future. When you want to withdraw from your bitcoin investment portfolio it's good that you read this
thread to understand how to withdraw once in a while to take little profits.
The market is filled with uncertainties, which makes trading, a gambling practice because in future, you will be left with very little or no bitcoin in your portfolio. Think big and be a long-term investor because the price of bitcoin will reach a price beyond our imagination in future.