In the space of 2 minutes, Bernie is booed by, what appears to be, supporters from rival campaigns; before being greeted by a throng of hundreds of supporters, who sang HAPPY BIRTHDAY to him. Bernie's birthday is today and this was recorded yesterday at the Milford, NH Labor Day parade.
As for who was doing the "booing" I have no idea (while I shot the footage, I was way to busy framing Bernie as he walked and I walked backwards). But the only people around him were supporters from several campaigns who were lining up for the parade.