Pretty simple, really. I think 2016 finally shocked the Millennials and got it into their heads that if you don't go out and vote, someone you don't like might get in, so they went to vote straight Dem. Throw in some illegal voting in certain races and you have yourself a real easy time flipping some house races, especially when you consider you're only dealing with smaller districts where a little fraud or higher turnout has a greater chance of making the difference. Trump knew the Republicans were likely to lose the House, which is why he never bothered to focus his rallies on House races. He focused on the Senate races, which he knew were more important and easier for the Republicans to win.