The Clinton Campaign hired Atlas Research who makes recommendations to the campaign and the media for how to get the desired result from polling by targeting certain zip codes, minority groups, Spanish only speakers etc.
Why rig a poll? Rigging a poll to make news is desirable for the “band wagon” effect. If people think that “everybody wants Clinton” many people will not want to feel left out, alienated or different so they get on the band wagon.
Thanks to WikiLeaks we know that dozens of elite media journalists are actively coordinating with the Clinton Campaign (2, 3, 4), this is why it is no surprise that so many polls sponsored by the elite media oversample democrats or poll people who aren’t even going to vote to get the desired result.