Simply put, it's a new slang term for photo error. More precisely, a photo that has captured something that was not intended. At first it was of the accidental type, but more recently as people have become aware of the term, more incidents of photo error are not of the accidental type, but now they are of the intended pranks and stunts.
The fact that there is an over abundance of this media and its coverage should be expected. Today's Photobombers are all looking for that chance to gain some internet fame quickly, to go as they say...viral!
Yes, viral, and not as per se, infected with something, but to have spread far and wide by way of internet sharing through the many social media outlets that outrageous photo that is sure to get noticed and talked about.
The unintended actions or things in photos are not new, they have been with us as long as photography has existed. The difference today is that we're more focused on them , and the new term we've given them.
Now there is an overload of Photobomb/Snapshots. These are mostly from smartphone cameras and other devices. A number of websites include sections on them entirely devoted to Photobombs.