This is a video of Mikhail Shivlyakov performing a 426kg (939-pound) deadlift at 2018 Arnold Strongman Classic and getting a gnarly bloody nose in the process. Somebody in the Youtube comments said it's a defense mechanism to protect your brain when your blood pressure is too tall, although I have no clue whether that's actually proper or not because it was a Youtube comment and not an entry in a medical journal.
Keep going for the video unless blood makes you queasy (I fainted twice).
Source: geekologie.com