Some random guy on Twitter called him a bedbug and Bret wrote an entire article about how that was antisemitic. It got enough attention that Trump made fun of him.
It came from an article about how the nyt building was being fumigated for bedbugs and someone on Twitter replied to it with a joke about how Bret Stephens was being taken care of or something along those lines. He read it and got incredibly upset and tried to ruin this guys life.