Wiktionary
n. (context Internet English) The practice or phenomenon of making an unexpected appearance in a video, especially with the deliberate intention of ruining it. vb. (present participle of videobomb English)
Wikipedia
Videobombing is the phenomenon of an unexpected appearance in a video of individuals who were not intended to be there by the operator of the camera or the individuals being filmed. The act of inserting oneself into someone else's video is often done in order to play a practical joke on the cameraman or his subjects, and sometimes in a deliberate attempt to create a video that could go viral.
For example, individuals wishing to videobomb someone may insert themselves into a video of a famous athlete or a celebrity being interviewed, or a live news broadcast being filmed in the streets or in another area where outsiders are present. Some people do this repeatedly, such as Chris Bosh and Paul Yarrow.
A related term is photobombing, which applies the concept to a still photograph instead of video footage.