They're pissed because they think something is wrong and they want it fixed. But they don't know what is wrong, and they don't know how to fix it. So I think they are pissed because it just seems like they should be.
> It could perfectly be due to rational reasons.
If you ask a very loud shouty person why they are so angry, they usually launch into a tirade of "everything that's wrong with the country." But you don't hear one single specific issue or well-thought-out rationale. The only common thread is cultural or political. Us-versus them; it's all those people's fault that everything is wrong. There are common themes, but no one specific root cause, issue, or analysis.
So they don't really know what's wrong or what the cause of it is. The only thing they're sure of is that something is wrong, and they grasp at whatever has been pissing them off lately. As social primates, when we are generally pissed off and think something is wrong, the most common group that we blame is "the authority" (the next-most-common group to blame being "the foreigner")
The general evidence I give is here (https://news.gallup.com/poll/1600/congress-public.aspx). In the 1970s, around 50% of Americans disapproved of how Congress did it's job. For the past 10 years, around 80% of Americans have disapproved of how Congress did its job.
Congress is an elected body. If the people really thought Congress was the cause of their anger, they could just vote them all out; doesn't take a genius to figure that out. Yet most people stay in Congress. So I think the anger at Congress is really just a symptom of the malaise, regardless of whether Congress does either a good or bad job.