It will likely take months to determine all the factors, but for some early analysis see:
What went wrong with the 2016 polls?
http://www.theatlantic.com/politics/archive/2016/11/what-went-wrong-polling-clinton-trump/507188/
The polls missed Trump. We asked pollsters why.
http://fivethirtyeight.com/features/the-polls-missed-trump-we-asked-pollsters-why/
Why 2016 election polls missed their mark
http://www.pewresearch.org/fact-tank/2016/11/09/why-2016-election-polls-missed-their-mark/