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.
Fluctuation and volatile are different with volatile price is more challenging than fluctuating price because the first one has wider price change range while the latter one has narrower price change range. You will feel more uncertain and panic with volatile price than with fluctuating price changes.
Investors who are new in this market will feel more stressful it's hard for them to control emotion as well as confidence in volatile days. Volatility triggers emotion and psychology consequently can be affected considerably from which there will be more chances of mistakes to be made by newbie investors. It is more extremely dangerous if newbie investors don't have good capital management at beginning and their investment portfolios have risk of liquidations or loan expirations.
Imagine that they might have to sale off their bitcoin even they are not panic, but it's because of a loan expiration and they need money to finish their loan or at least pay out loan interest before they can be able to expand their loan for a longer time.