Some aliens got fed up with the humans on board. So, they set up speakers to make loud or ominous noises every time a human walked near. -- Anon Guest
Psychological warfare has been banned in the Galactic Alliance, and those who wish to bring it back are quickly educated about the Whisker Incident. It was, at least in the beginning, a study on crew incompatibility, but like most social experiments of a certain nature, it escalated too quickly and got out of control.
The crew of the Twitching Whisker didn't much like their ships' Humans and the Humans didn't like them. Initially, there was a period of attempted pack-bonding but, when that failed, the mood of the crew turned sour. Hostilities almost turned to fighting, but the breach of contract clauses were too strong to allow the Humans to harm the rest of the crew.
The others aboard, a diverse sample of Maunchini, decided that they would be better off if they could somehow convince the Humans to leave at the next port and never come back. Thus began their fear campaign. The Maunchini did their homework about making the Humans feel uncomfortable, cursed, or uneasy about their environments. The exact amount of lux necessary to render any environment creepy or depressing. The 'perfect' amount of background EM radiation to cause a Human to very subtly hallucinate and doubt their senses. Infrasound that impinged on Human instincts to run and hide from the monster right behind them...