It is hard to always know what happened, but frequently if someone invests too much then they are expecting the price to go down and not prepared for it to go down, and surely there can be advantages to the BTC price going down, even if you had preferred for the price to go up, which is buying more when it goes down.
So it can take a long time to actually get ahead of your investment in order that the price is always coming to you rather than you going to the price, so if you just continue to buy in a DCA kind of way, that can be one way of getting ahead of the BTC price, even if it might take 2-3 years to actually get to a point that your cost per BTC is actually less than the current price. It is not guaranteed to happen, but so far anyone who persistently buys BTC, has been ending up getting into profits, even though sometimes it could take 2-3 years before the "being in profits" stage can actually end up happening.
A person who panics, and does not continue to buy but then sells and buys will likely NOT really get to any kind of stage in which his./her holdings is in front of the price.. and it seems such a simple strategy to merely just continue to buy (sure buying on the dip is fine, but buying at any price might well end up being fine in order to keep a person in the right kind of mindset of continuing to build the number of BTC that s/he has in his/her holdings and keeping some faith that that BTC is going to be profitable in the future.. even though it is not guaranteed, it has been a pretty good place to put money and likely to continue to be a good place to put money.
Is it a form of risk that accidentally becomes a burden on their minds so that they are unable to survive when the price of Bitcoin drops significantly? Indeed, investing large amounts of money or doing it all at once will affect their minds because they do not yet have a strong mentality when the situation turns around to what they expected.
In that context, investors who start with the DCA strategy may be better prepared when the market situation changes suddenly because they can buy more BTC and those who are not mentally strong may be quite stressed by this situation. However, the essence of this point is of course that long-term investment planning requires many things that must be prepared in advance, such as being mentally ready to face all changes that occur at unexpected times. Being a loyal holder of Bitcoin requires strong mental strength to be able to carry out what has been previously planned.
Many times there are trade-offs, and even though DCA tends to be the best, DCA might not be the best for a person who has a lump sum of money currently available, and it might be better to lump sum most, if not all of that money, and then perhaps just have money to continue to buy if the price dips.... that is why a newbie to bitcoin likely needs to consider the three categories of 1) DCA, 2) Lump sum and 3) Buy on dips, and perhaps have some money allocated to each of the three categories, and if s/he chooses to eliminate any of the categories, then s/he would have at least weighed the pros and cons of each of the categories.
So a person who suddenly has $6k available could divide the $6k into 3 parts, and then maybe also consider what to do based on considerations of anticipated cash coming in too.. so if the next 6 months, the person also has $6k of extra cash coming in, then that is going to be $12k over the next 6 months, and then another consideration might be how many BTC does s/he already have. Maybe if s/he had already been buying bitcoin for a year then the perspective would be different from a brand new investor, and even as compared with someone who might have been buying BTC for 6 years or more. How highly DCA would factor into the various situations is going to likely play out differently, especially if we also include some of the other factors such as view of bitcoin as compared with other assets, which other assets are already owned, risk tolerance, timeline, time, skills and abilities to trade, reallocate and/or to learn more as things go along.