A large part of the problem here is that you are viewing the chanters as Donald Trump supporters. Perhaps that's not how they view themselves. Perhaps they view themselves as protesters who are bringing attention to the ongoing failure of the government to enforce proper treatment of classified information.
There is an argument that Hillary Clinton should be arrested and charged. She was given classified information that she then treated recklessly and carelessly, with some officials arguing she violated federal law. The decision not to charge her was made by someone who himself leaked classified material.
Another observation that the chants may be less about supporting Trump and more about Clinton:
But what struck me as odd, in retrospect, was that this man was still obsessing over his hatred of Clinton, nearly two years after she lost the election. He had nothing positive to say about the man he’d voted for, only wrath for the woman who ran against him.
Really though, if you want the answer to this, we're the wrong place to ask. Go to a Trump rally, find a chanter, and ask that person. Repeat until you get a nice selection of answers with many repeats. The only thing we could really do would be to link you to someone else who did that. But it seems like the media is more interested in speculating than in doing actual research, as I can't find anything like that.
