Zach Braff was walking through New York City when he happened upon a just-married couple exchanging kisses in the middle of the street in front of a photographer. So he did what anyone else would do and photobombed the snap. That'll be a fun story for the grand kids!
Merriam-Webster defines the term "photobomb" as ... well, absolutely nothing, because photobomb has yet to be added to the dictionary. It's more one of those "you know it when you see it" sort of things.