LA Times had an insightful article on the topic: "Polls may actually underestimate Trump's support, study finds", which contradicts the accepted answer's theory to an extent, and is much closer to your question's theory #1 (Having said that, I agree with @bobson that at this point we probably don't have enough hard data to be sure what the causality is).
The study (by Morning Consult, a polling and media company) found that, when randomly choosing which method to use to poll individuals:
... confirmed that "voters are about six points more likely to support Trump when they’re taking the poll online then when they’re talking to a live interviewer,” said Dropp.
Some significant number of Trump supporters, especially those with college educations, are "less likely to say that they support him when they’re talking to a live human” than when they are in the “anonymous environment” of an online survey, said the firm's polling director, Kyle Dropp.
The most telling part of the experiment, however, was that not all types of people responded the same way. Among blue-collar Republicans, who have formed the core of Trump's support, the polls were about the same regardless of method. But among college-educated Republicans, a significant difference appeared, with Trump scoring 9 points better in the online poll.
The researcher posed a plausible theory to explain the discrepancy, which neatly addresses your own conundrum:
The most likely explanation for that education gap, Dropp and his colleagues believe, is a well-known problem known as social-desirability bias -- the tendency of people to not want to confess unpopular views to a pollster.
Imagine your own conversation with your peer, 2 versions of it. Importantly, the "You" quotes are ones that your peer imagines you will make (after hearing all the mainstream conservative Trump-disapproval and mainstream liberal media Trump-bashing, or simply talking to another anti-Trump Republican before) - NOT necessarily what you will actually do or say.
Imaginary You: "I'm voting for Rubio"
Social-approval-conscious-Peer: I am voting for Trump
You (looking as if they just admitted to liking Tila Tequila): "But Why? He's a Misogynist!"
Peer: No he's not. He's acting like an %$$ to everyone of any gender.
You looking at your peer with disappointment or unliking them on Facebook
You: He's a racist! He wants to make Muslims wear a badge! He's Godwined!
Peer: For a professed conservative, why are you so eager to believe liberal media lies that are easily disproven?
You: looking at your peer with even more disapproval
You: "I'm voting for Rubio"
Peer: says anything except they voted for Trump
You: Not disapprove of peer
As you can see #2 is clearly a superior outcome for your peer.
UPDATE 2016/11/09: [FiveThirtyEight] in their post-election analysis also noted similar effect (thought not every pollster seems to agree) in general election:
Several pollsters rejected the idea that Trump voters were too shy to tells pollsters whom they were supporting. But James Lee of Susquehanna Polling & Research Inc. said his firm combined live-interview and automated-dialer calls, and Trump did better when voters were sharing their voting intention with a recorded voice rather than a live one.
Women who voted for Trump might have been especially reluctant to tell pollsters, said David Paleologos of Suffolk University. The USC Dornsife/Los Angeles Times poll corroborated that: “Women who said they backed Trump were particularly less likely to say they would be comfortable talking to a pollster about their vote.”