My impression: staying home is a way of voting for some people. There are some (many?) people who have so strongly identified with one party, that when they are upset with it and want to "vote against" it, they still cannot bring themselves to vote for another party, as that seems a violation of their core identity. So, staying home is how they vote for the other party. It only counts half as much, of course. So, in this example, there might be some black voters who cannot bring themselves to vote Republican, no matter what, but if they are upset with the Democratic party they may stay home. Similarly, Christian conservatives who don't like the Republican candidate may not be willing to vote Democratic, but they might be willing to skip voting.

Just a hypothesis, of course, I have no data to back that up.