It is in response to a high volume of online harassment and crude comments toward Clinton and her supporters (by a small and extremely vocal minority of Sanders fanatics).

It is a lamentable that the vast majority of Sanders supporters do not behave this way and don’t deserve the label.

Yes, but it's used by his opponents to his right within the Democratic Party as a guilt-by-association attack to make those in the electorate on the left uncomfortable with him, it's not actually an attack from the left.