Edit: comments has suggested what I've learnt is wrong. Read comments for correct Info.

I observed this while my latest encounter with him. Time was running out so tried to get near him to make all my shots count as Talus. But got scared away twice. I've been near Fernando and Vivian bosses without the feared effect. This might be specific to Viktor Wolf Boss.

Edit: Because spawn of Full-moon Viktor is so rare, I can't replicate and confirm what I observed. If the boss spawns in match and you can confirm the effect, this will become a confirmed thing.

Edit2: A comment suggests the fear is from Viktor's grenade, and not by coming close. That makes sense too.