I don't understand the details of it but what happened as far as I know was this:
They get contracted by Bethesda to produce a game.
There is a framework that they work on to make Shelter work the way it does.
That framework is Bethesda's intellectual property now.
They are not allowed to just reuse it.
They go and get hired by whoever owns Westworld.
They use the framework.
The framework has a bug in it that is unique to it.
That bug is what caught Bethesda's full attention and they started to probe further into it.
And saw that hey this is ours!
And then Bethesda sues.
I suppose it's the programming equivalent of Bethesda copying a DnD story for its game Elseweyr.