1. Someone uploads funny/interesting content.
2. Other people (bots maybe) notice that it's popular and download and re-upload it, titling and promoting it so that it ranks above the original in search results. Often the re-uploaders file takedown notices against the original video or other re-uploaders, so they're removed.
3. Repeat step 2 hundreds of times, until any search for "funny/interesting content" returns only a series of almost-identical 240p 15fps cropped, mirrored, or otherwise unwatchable versions of the original video.
I don't know if they condone this because it results in more overall views, or if they're just terrible at fighting it, but it's largely made Youtube useless for its original purpose as a many-to-many video sharing platform.