We don't really know yet. There are two (quite likely) possible options. A shift in the voter demographics and/or the "shy tory" (people not admitting to vote conservative because a negative social consequences).

Polls are never truely objective even if they try to be. You always have underlying assumptions about how to get a representative sample.

The third option would be that the polls were rigged. After reading the leaked Podesta mails this doesn't seem too unlikely but still less likely then other possible reasons.

Because it polls likely voters. The unlikely voters came out and voted.

The LA times poll did not get it wrong.

FBI effect was not captured in the polls

extrapolation