The chart is broken up into seven different categories: transportation, industrial, electric power, commercial and residential, agriculture, high GWP and recycling and waste. Industrial and electric power dropped the most precipitously in this period; the San Francisco Chronicle notes that the latter is due to an uptick in solar electricity generation and hydroelectric power, which correlated to a 15 percent decrease in using natural gas for electricity.
However, some categories continued to rise. Most notably, despite California's incentives for purchasing electric vehicles, greenhouse gas emissions from passenger vehicles increased, which means that people are purchasing more gas-powered cars (or just driving them more than they were in 2015). This is likely due to low gas prices and higher prices on EVs.
This report certainly gives us some hope for the future; despite the fact that the administration refuses to acknowledge climate change exists, it shows that states can pursue their own path towards cutting greenhouse emissions. California has already mandated that emissions must drop an additional 40 percent by 2030. This will be a much larger challenge; as the San Francisco Chronicle points out, emissions must continue to fall as much as they did between 2015 and 2016 each year, or they will miss the goal.