The reason was offensive tweets that Brian Waggoner wrote in 2010-2016. They surfaced last weekend, after which some users accused the developer of racism and homophobia.
Crystal Dynamics apologized to gamers for the inappropriate behavior of an employee and announced that it had removed him from the position of an official representative of the studio.
Waggoner also made a public apology, assuring that he had changed for the better over the years.