Happening by chance; coming or occuring unexpectedly, or without any known cause; chance; as, the fortuitous concourse of atoms.
source: Webster 1913
adjective satellite
having no cause or apparent cause
A causeless miracle. Fortuitous encounters--strange accidents of fortune. We cannot regard artistic invention as...uncaused and unrelated to the times.
source: WordNet 3.0
occurring by happy chance
Profits were enhanced by a fortuitous drop in the cost of raw materials.
