A sudden decline may be because viewers are no longer interested in the topic of the video (this happens a lot with trends, viral videos, and news). That's why it's so important to create evergreen content (open article). One reason for suddenly losing views is that the new videos simply push the winners for a few days. If your older video gets a lot of views from Explore, on the viewers' home page, any new video promoted by YouTube will compete for those views.
YouTube videos can lose views from YouTube Audits. For videos that start to accumulate views, YouTube conducts audits to rule out dubious accounts. Auditing is an ongoing process, and YouTubers should expect fluctuations in views over time. If you pay attention to the view counts for videos on a YouTube channel you own, you might expect them to be relatively stable.
It's also where the most distress occurs when that video, the one that generates a third of your channel views and subscribers, suddenly loses thousands of views per day. This could be higher, if you continue to get legitimate views during that time, or it could be lower, since a lot of fake views are eliminated. Some views will be added, others will be removed, and overall your view count will increase and decrease overall. Therefore, a video with 3000 views can gain more than a video with 10,000 views, and CPM makes the difference.
Keep in mind that very high view counts will have a much lower CPM to save advertisers costs. YouTube was tired of answering questions about the 301-view hiatus and they were tired of having to deal with the problems it caused. Views that come from a user who only updates a browser over and over again, or views that are purchased from third parties do not count and are deleted. The best way to avoid losing views on your YouTube channel is to constantly produce new, high-quality videos.
The problem with this came from people watching their watch counts in real time or simply monitoring their video views on a daily basis. However, it's important to recognize the different reasons why YouTube views drop and distinguish between things you can control and things you can't. YouTube was fined hundreds of millions of dollars and had to fix it and, in extension, this caused a lot of unexpected changes for YouTube creators. They would analyze the logs and behavior of both the old views that were registered and the new views that arrived while auditing everything.
Over time, repeat views will no longer add to the view count of the same viewer account after a certain number of replays. However, to maintain a smoother view counting experience, YouTube has decided to conduct ongoing audits and validations.