I don't know. Could it be that they are angry young people with not very good impulse control? I certainly don't approve of such action.
It is notable both that you bring up the incident and suggest that there might be some reason such an incident should be held against Democrats. You don't make the effort to advise people not to conflate such moronic behavior with Democrats. For example, you might have
referenced Nancy Pelosi's statement on Antifa from last year
“Our democracy has no room for inciting violence or endangering the public, no matter the ideology of those who commit such acts. The violent actions of people calling themselves antifa in Berkeley this weekend deserve unequivocal condemnation, and the perpetrators should be arrested and prosecuted.
“In California, as across all of our great nation, we have deep reverence for the Constitutional right to peaceful dissent and free speech. Non-violence is fundamental to that right. Let us use this sad event to reaffirm that we must never fight hate with hate, and to remember the values of peace, openness and justice that represent the best of America.”
You might have contrasted that with Trump's approach following Charlottesville. It is Trump's "even-handedness" in equating murder by Nazi racists with property damage and scuffling by counter-protesting Nazis that was alarming even to mainstream Republicans. You might have said this is one of the reasons why Trump and Trumpism needs to be purged from the GOP.