Every writer is different, but for me, I like to have a lot of balls in the air throughout the entirety of my books. Having said that, there's something about that halfway point that screams at me to throw another curb ball into the game to see what happens. I feel that it's usually the perfect time to add an extra element to completely stress out the protagonist, therefore creating excitement for the reader.
Of course, this also means more of a challenge for the writer. But you're up for that. Right?
For example, in my book The Devil May Lie, I threw in a political scandal at the halfway point, causing a shift in energy for the story. This created a heavy, dark feel in the book. However, it was necessary to shift gears and move the story along. It was interesting to see what direction it took the characters and how everything unraveled.
It's important to keep readers on their toes. But of course, as a writer, it's not a bad idea to keep on your toes too.