The astral and mental bodies are the repositories of the mind, thus they are damaged by psychological impacts, not physical ones.
Damage to the mental body can be caused by abusing the intellect (ie. Nietzsche), obsessive thinking or analysis, through fanaticism (fascism, religious fanatics, political fanatics, etc), addictive behaviors (any fixated, obsessive behaviors, like watching too much tv, constant internet use, video games, etc), and many, many other ways. It is damaged by alcohol, drugs, sexual fantasy, sex addiction, and masturbation. Certain black mystical or spiritual techniques (done in this life or the previous one) damage the mental body.
Oftentimes we do not want to recognize that we have done something wrong, so we do not "think" about it: this separates intellect from emotion. We "compartmentalize." This principle illustrates how when a person has engaged in witchcraft, black tantra, killing, raping, abusing others, etc., these severe actions can "split" the astral (emotion) and mental (intellect). The person becomes very divided internally. Again, this has symptoms of "multiple personalities," etc. Of course, this is also facilitated by alcohol, drugs, misuse of sexual energy, etc.
"Do not worry; cultivate the habit of being happy." - Samael Aun Weor