You may not think about it, but travel, especially international, can be a very stressful experience. Travelers get separated from their familiar social support systems, and must adapt with foreign cultures and languages, and deal with unfamiliar threats to health and safety.
Coping with such a high level of travel stress could stir up physical, social and psychological problems. People who get to encounter a greater range of stress factors are more likely to bite it from psychological problems. Due to the stress of travel, pre-existing mental disorders could be exacerbated.
More to these, for individuals who have a predisposition towards rmental disorde, that disorder could surface for the first time during travel.