When a hopper minecart is below and to the side of another hopper minecart that is turning, it has the potential to take items from the hopper minecart that is above.

Video evidence: https://www.youtube.com/watch?v=c0QJobyD-F0

Edit: After continuing to try and find work arounds to this problem, I also found that hoppers next to the turn also can pull items out of the hopper minecart.

Video Evidence: https://youtu.be/bIAjklb8eAA