I believe the bro’s represent a certain specific sub-group of Bernie supporters. They tend to be white men from middle class backgrounds who usually have some sort of community college education that they have failed to complete. They are known for being angry, hyper-aggressive guys on social media who pick fights, make misogynistic comments, doxx their enemies, etc. They are known to be a particularly nasty bunch of characters.
Not all Bernie Supporters are like this. They are probably a minority...maybe 30% of his base. Maybe comparable to the percentage of Trump Supporters who are hardcore, 2A, white nationalists.