Bethesda is taking HBO's parent company, Warner Bros. to court. Their grievance is centered on the belief that the Westworld game is using code from Bethesda's Fallout Shelter.
Bethesda is primarily known as a studio but they're also a pretty active publisher of video games. Back in 2014, they hired Behaviour Interactive to work on Fallout Shelter, the studio that WB would go on to hire to make the Westworld game.
It seems that whatever contract Behaviour Interactive signed with Bethesda, the latter believe it gave them ownership of the code. Thus, the use of it to make the tie-in Westworld mobile game would be prohibited.
Bethesda believes the same code is being used as the same bugs and issues in the Westworld game were present in an early version of Fallout Shelter. Bethesda is seeking a share of any profits and for the Westworld game to be shut down.