My gut says aggression, too. I just went through something very similar w/my FW puffers. The 3 grew up together in their current tank and were fine for like a year, but now the male is mature and big, and he's a bully. Almost killed one of the females, and it came to a head when she was behaving similarly to how you describe. I had to put her in an isolation box where she got better w/in a month. Then I let her out and put the bully in the box for a couple weeks. Rescaped the tank the same day I let him out, and now we'll see. Fully prepared for needing to rehome him or the females tho b/c he thinks the whole tank is his.
I would try to stake it out and see if you can catch any aggression towards the watchman.
I would try to stake it out and see if you can catch any aggression towards the watchman.