I think Trump draws a lot of broad support from his 'anti-establishment' rhetoric so it kinda makes sense for folks to look to other anti-establishment candidates as an alternative to him. There's a tendency to look down upon Trump voters as only right wingers, conspiracists etc and not really understand that a lot of his support is superficial and based on limited information.
In a way it's not so much that Stein or other left candidates are spoilers for the establishment Dems but more the case that figures like Trump are spoilers for progressive alternatives to the establishment.
You mean like SELinux or other existing contributions to the linux kernel?