His reasoning is that there are just too many threats for humankind on Earth, including a global nuclear war, genetically modified viruses gone rogue, and of course climate change. Over-population and threats from space (like asteroid impact) are among the dangers of secondary likelihood, according to Hawking.
There was a time when Hawking used to believe humankind would not be able to establish space colonies that could sustain themselves without help from Earth. But now his opinion has changed: we should find a way to do it. His main concern is that people would not take the problem seriously enough to start working on it until it is too late.
Hawking will make the documentary together with his former student Christophe Galfard. They have toured various spots in the world in search of technologies and ideas for creating a real, long-term, self-sustainable colony on another planet.
This is not the first time Hawking has issued such stern warnings. He is among the most vocal advocates about the risks from developing AI. He believes that if we are not careful enough, technology could bring the end of humankind.
...Probably way before plastic-eating worms do.