This might seem surprising, but it stems from the ancient belief that stars influenced the course of human events. When a disaster occurred, it was often attributed to a negative celestial influence.
Here's how the word evolved:
* "aster" (star)
* "dis-aster" (ill-starred, unfavorable star)
* "disaster" (an event causing great harm or damage)
So, the original meaning of "disaster" was something like "a misfortune caused by an unfavorable star". Over time, the connection to the stars was lost, and the word simply came to mean a catastrophic event.