"Buy the rumour, sell the news" is a very common adage in trading. DOGE may have shown us one of the purest examples of this. DOGE has been an extremely bullish trend for some months as buzz around the coin continued to grow. As it started flailing, it was rumoured that Musk might mention DOGE while hosting SnL. This caused the price to go even further - topping out around $0.72. While many investors thought that Elon might bring even more gains to the stock - the opposite happened - and as soon as he mentioned DOGE, it took a very sudden and drastic turn.
The psychology behind sell the news events
When something is highly anticipated, or a "rumour" is surrounding an asset, investors will flood in, expecting to be able to cash in at a profit (or even hold longer term) after the rumour becomes official information. This will cause the asset price to rise, usually beyond a point that is reasonable - to a point where even when the rumour becomes concrete, it doesn't really add any value to the already inflated price. Then, once the rumour becomes reality, investors realise that nothing of value was really added - because the rumour has been "priced in" - this means that investors who were anticipating a rise after the news about the rumour (especially short term investors) will quickly bail. This can often send the asset in freedive as it starts seeing a wider selloff, especially in crypto.
At first, DOGE got buzz for miscellaneous reasons. It was a joke coin, and kind of just broke out from a long period of price consolidation earlier this year. Early money (or "smart money") saw an opportunity and started buying. After that, it got onto the wider public's radar, including Elon Musk who started pushing it (again) on Twitter. The momentum built and built, and DOGE started an insane bull run that saw it rise tens of times over. As an asset becomes more and more overbought, you need to bring more buyers in, generate more insane buzz, or ideally, create more intrinsic value. Of course DOGE has no intrinsic value, so that's not an option. What happened instead is that Elon Musk hinted at DOGE mentions during an SnL episode - this sent the asset soaring once again. But as you may have noticed, this time the rise didn't have quite the same oompf. Once it actually got to the show, at the beginning, some people bought in anticipation, but then as soon as the asset was mentioned - boom, the price took a deep, deep dive.
This happened because there was really nothing more to say about DOGE. There was really nothing more Elon could say to give DOGE more value (maybe he could have mentioned Tesla stockpiling it, or accepting it as a payment method - but this was extremely unlikely.) Everyone who was going to buy had already bought, and since the anticipated event (the "rumour") had become reality (the "news"), the only thing that was left to do is sell.
Does that mean all anticipated events lead to falls?
Not at all. For example, tech companies often announce the launch of new products. There may be rumours surruonding the product, and then depending on how the actual launch event goes, the stock could fall or rise. In many cases, it falls, because there are often a lot of expectations, and launches are often a little bit underwhelming. This is especially true for phone companies - who let's face it - don't really add much to their phones with each year these days. This is what happened with Apple's unveiling of the iPhone 12 last year. The stock rose before the launch event in anticipation, and then when it actually happened, there was a small dump which led to a fall over a longer time period.
However, if the launch event goes really well, and knocks everyone's socks off - for example if an unknown feature is announced, or they do something revolutionary - this can cause the stock to soar still. But these cases are really the minority. Generally I will look into options trades before launch events in stocks to try and capitalise on this relatively risk-free. I may make an article about that later.
You can assume that most events are going to be underwhelming, and that the people who bought in anticipation are probably going to sell after the event - whether it rose or didn't. And it probably didn't. Thus we say: "buy the rumour, sell the news."