Trust in government has been low over the two decades shown in the graph. It increased in the Clinton administration, peaked in 2002 as the government organized a response to 9/11, and decreased thereafter going to a low value early in the Great Recession.
The legislative branch is producing less legislation these days than in previous decades. Perhaps that is reflected in the falling confidence levels. The massive use of negative advertising in election campaigns, and the rise of cable news networks that are often on the attack may also explain some of the fall in confidence in government.
The results shown in the publication shown don't disaggregate data by specific function. Perhaps there is more confidence in some functions than in others. For example, I have considerable confidence in NIH, CDC, and the statistical offices of government. I have less confidence in some other agencies.