This is just one of three visualizations from NYU's Stern Urbanization Project, which is looking at patterns of how cities grow over time.
Over at The Atlantic Cities, Richard Florida explains what you're seeing above:
And in Los Angeles, we can see the multiple nodes of growth that defined the sprawling region for much of the twentieth century. Growth took off first around World War II, and the city expanded west during the 1940s. During the 1970s and 1980s, the spread of orange across the landscape is a signal of the rapid sprawl that many associate with Southern California.
Los Angeles is particularly interesting because this very growth pattern has been the subject of countless dystopian stories, from the work of Philip K. Dick and a recent Judge Dredd comic series about Mega City Two (AKA the LA sprawl), to Neal Stephenson, who describes the future Los Angeles region simply as The Burbclaves in an early novel.
One of the first apocalyptic novels of the twentieth century, Nathaniel West's The Day of the Locust, is about the hideous world of Los Angeles in the 1930s, when the cities tentacles had only just begun to stretch themselves (there are a lot of references to that novel in the movie Barton Fink).
This map, with its city that seems to grow like a cancer, reveals why Los Angeles has haunted our dystopian dreams.