People with dissociative disorders will sometimes involuntarily switch personalities when exposed to certain stimuli. The required stimuli varies from person to person. Typically, however, triggering stimulus is anything that forcibly reminds the subject of their own experiences of being abused.
In my own personal experience, fragmented Multiples are fairly easy to trigger, co-conscious Multiples are more difficult, and integrated Multiples are very difficult to trigger.
Because my own story involves being on the receiving end of quite a bit of abuse, I have posted this warning.