Sure. There’s a lot to it, so I’ll copy/paste (with some minor modifications) an explanation from elsewhere.

It started when a YouTuber whose channel is called Coffee Break reached out to Philip of Kurzgesagt as part of a video he was doing into the flaws of popular science communication. Specifically, about some significant errors in K’s video on Addiction. Instead of agreeing to collaborate, or even giving a simple “not interested, sorry”, K took an instant accusatory tone, claiming CB must have been making a “gotcha” piece. CB and K agreed that they would talk more about the matter to try and assuage K’s concerns, but K kept stalling while working on a retraction video, at which time K took down the video that was the impetus for this discussion (shortly after, as one of those aforementioned stalling efforts, having said “I never could bring myself to take it down”, claiming it would be “cruel and unnecessary” to do so—funny, considering in his AMA attempting to spin the story, he said “I was really stressed out about the addiction and the refugee video for years. Being finally open about my mistakes and deleting them felt like weight leaving my body.”). The Refugee video was also taken down along with the Addiction one that CB was interested in.

K claims to be interested in science communication. But here, he decided to make the selfish decision to do what he thought would protect his own personal brand through duplicitous means. He got ahead of the story that falsely assumed was coming, and put up a pre-emptive response to that. Now, CB isn’t entirely blameless. In response to the above, CB put out a rather hot-headed reaction to the whole incident. He didn’t follow up with K to try to understand what had happened; he lashed out in anger at K’s self-righteous arse-covering video.

And then CB started getting harassed. K called out CB, and many of K’s friends (other very large, powerful YouTubers such as CGP Grey and Philip de Franco) made very public statements to their audiences attacking CB. It ended up forcing CB into taking down his video, deleting a whole heap of tweets explaining what happened, and putting out an apology. Perhaps it was an apology that CB should have indeed made, but the need for an apology from K was much, much greater. And one never came. K used his larger platform to spin the narrative so that his large audience, and now also the general public who becomes aware of this, almost all take his side.

Incidentally, here’s the video that CB was working on at the time. Hari, the scientist discussed in the video whom K worked with on his video discussed earlier, communicated very well with CB on it.

And as a separate thing that came out a while after this, Both Kurzgesagt and CGP Grey were early members of Nebula. Today, Nebula is fairly well-known as a creator-owned streaming service. Creators literally have an ownership stake in the business, and are paid out a slice based on their viewership (I don’t think the exact formula is public, but presumably some function including number of views, view hours, etc.).

But early on, when it was still taking the shape we know it in today, Grey and Kurzgesagt were members with an ownership stake. They left it a long time ago citing “creative differences” (or some vague nonsense like that). From little pieces that have come out on the rare occasions that existing Nebula creators have said something about it, apparently those two were less interested in the vision of Nebula as a place where multiple creators support each other in growing their collective revenue; instead they wanted to take more of a parasitic approach where they could profit off of the backs of other, smaller creators. Because the then-co-owners weren’t interested in that, Grey and Kurzgesagt ended up leaving.