There were six congressional races with 2 Democrats and two with 2 Republicans in the November 2012 top two elections. In addition, there were three races with a Democrat and an NPP and one race with a Republican and an NPP.
All of the races with at least 10% drop-off didn't have a Republican and a Democrat running. While some in the opposing party will decide between the "lesser of two evils," others will leave the choice blank. That makes it very difficult for candidates. How do you know which voters in the other party are going to vote and which will leave your race blank? You have to sell the first group on you, but don't need to bother with the latter group. Is it best not to bother at all?
If there's a top of ticket race that'll draw people to the polls, the candidates will need to pay some attention to the other party's voters. The LA mayoral race will be the top of the ticket draw, however. The drop-off in November indicates Republicans won't show up to vote for either Democrat.