What I think happened is the film series was stuck in limbo after the last film didn't do that well, he had an idea, and Tarantino agreed to put his name on it as director to drum up publicity and help it get off the ground.
If it actually does make it to production I'll bet they'll pass it off to someone else. And rightly so because I can't think of someone whose sensibilities are more at odds with what Star Trek should be than Tarantino. It's bad enough that they said "f***" in the new series!!
As for Django, I've only seen it the once but it felt to me by far his sloppiest script. The oscars for both him and Waltz were daft. I thought Hateful Eight was better.