This article explains it pretty well.

There's just a ton of stuff and it all came out of the woodwork once the chips started falling. I first became aware they existed over the Billet Labs nonsense.

From the article:

The controversy started earlier this week, when Gamers Nexus posted a video outlining a number of factual errors and ethics concerns in recent Linus Tech Tips videos. “We’ve been seeing an alarming amount of conflicts from Linus Tech Tips as it relates to their corporate connections, their flow of money, and the potential bias as a result of those things,” said Gamers Nexus host Steve Burke.

Burke’s biggest concern was around a GPU cooling block made by Billet Labs. According to Gamers Nexus, Linus Tech Tips reviewed a copper cooling block from Billet Labs on the wrong GPU, then auctioned it off at a recent fan event without the company’s permission.