Why didn't they vote for President? Didn't want Hillary or Trump, or any other candidate?
Nobody knows.
Nobody audited those votes. Nobody surveyed those voters. No oversight bureau checked the hardware or software, exit polled, compared with absentee or other mail in votes, or in any other way vetted that vote count.
The recurrent statistical anomalies in key States of US national elections (of which that was merely one example) are discovered not by official or standard oversight procedures, not by investigative journalists from major media news outlets, but by political activists willing to do the work and bear the expense of overcoming the various legal and practical obstacles thrown in their way.
The voting machines and counting software were not third party audited in advance either - most of the corporations who provided them over the past few election cycles are owned and managed by large donors to the Republican Party, which in turn has legislatively blocked such security and transparency measures - citing the proprietary nature and protected market advantage of those corporate creations, and also the expense involved in vetting them.
In States with more thorough vetting practices - such as Minnesota, where I live, which mandates hand marked paper ballots which are kept after counting (a "paper trail", good for backup in the event of problems), audits a random sample of voting districts automatically, and so forth - those kinds of statistical anomalies are less common, smaller, and show no Party or demographic bias.
Meanwhile, such things are difficult to report on specifically - in terms of actual events - from a "bothsides" perspective, so the major media news coverage has been by necessity vague and largely focused on evaluating the Republican accusations of Democratic Party fraud, which being fictional can be fitted more easily into a bothsides narrative by balancing them against Dem denials: favorably toward the denials, which are more plausible, in more reality based and never-Trumper employing outlets such as CNN or MSNBC or the larger newspapers, favorably toward the Rep accusations in the more rightwing media such as talk radio and Fox and the propaganda feeds into Facebook.