Historically there's a strain of anti-Semitism that's gone along with some elements of African American political activism. I don't think we can rule this out as an explanation for why Sanders, the only Jewish presidential candidate running in this cycle, has become the primary target for these disruptions.
Anyway, the Secret Service shouldn't offer Bernie a choice after this. When people are literally shoving a 70 year old candidate off of a stage, his personal safety is obviously in danger. That cannot be tolerated.
This group also likely has backing from the Anonymous-affiliated radicals who are grossly anti-semitic and infiltrated Occupy, the Gaza protests, and Ferguson to instigate violence. There's definitely an element of that here.
And yeah, Sanders should accept Secret Service protection. Ideally some well-trained black ex-marines who can form a brick wall that the protestors can't get through.