How Mefloquine may induce psychiatric symptoms-
Mefloquine is an antimalarial drug. It kills the parasite that lives in the blood cells.
I am guessing it changes the microbe population in the body allowing another parasite to flourish.
Gastrointestinal side effects of mefloquine are well known.
Here is an article linking it to Pneumonia.
Mycoplasma pneumoniae is a pathogen which is known to cause sudden and extreme behavioral changes.