there are a number of reasons for withdrawing from such a study which may have nothing to do with the study and its efficacy or adverse events.

You can use your imagination to put up any one of unlikely but possible situations such as a death of a whealthy parent in another country with an active business which was left to a participant who feels it necessary to move to the other country to take over supervision of the business and arrange to either run it or sell it which because of the nature of the business requires a period of time.

in this scenario there are any of a number of complicating factors such as fixed assets which may require personnal attention such as a number of appartment and office buildings to be dealt with for sale and require personal attention

this is just one possible scenario wherein a participant would withdraw and pay attention to family needs.