David Hogg for anyone not aware. See DNC Vice Chair section.

Short answer is the young DNC vice chair who lived through a school shooting wanted to soberly look at the state of the DNC and was kicked out on some BS technicality.

After seeing a serious lack of vision from Democratic leaders, too many of them asleep at the wheel, and Democrats dying in office that have helped to hand Republicans an expanded majority, it became clear that Leaders We Deserve had to start primarying incumbents and directly challenging the culture of seniority politics that brought our party to this place to help get our party into fighting shape again.

Full letter here. I am leaving out good stuff.