Adventitious: associated with something by chance rather than as an integral part; extrinsic.

I didn’t realize until I checked the photo that all of those seagulls were lined up because my eyes were only focused on the waves. The gulls were an unexpected happenstance.

The term “line up” is usually associated with a row of surfers in a spot where they anticipate a good wave break, in their attempt to catch that elusive best ride of the surf sesh.

Adventitious; an unanticipated word of the day.