A wandering trader was attacked, he has no particle effects, i checked, to be sure he wasn't on speed pots. Now he is running around in an area of about 15x15 blocks, and he is going really fast, so fast he is causing a bit of lag. I kept right clicking waiting for him to pass by, and when i finally got to his trade menu it closed after a second, due to him already being too far away. Whatever was attacking him was dead before i got to him, but he just wont stop running!

     Correction it appears with a little more testing that if he is not immediately out of range, he will stop when you open his trading menu. But when you close it he goes right back to running.