Original: Personal unions can arise for very different reasons, ranging from near coincidence (a princess who is already married to a king becomes queen regnant, and their child inherits the crown of both countries) to virtual annexation (where a personal union sometimes was seen as a means of preventing uprisings).
Simplified: The case that a princess who is already married to a king becomes pregnant, and their child inherits the crown of both countries is a rather common  cause.