The act of one person interrupting another during a story told to a group, so as to let others know that they were present for the event, or in an attempt to steal the glory of telling a good story. This may happen at any time during the introduction, setup or punchline phases of storytelling.
"So anyway, we're sitting there, and this kid was passed out, and there was this plate-"
"Oh I was there for this, we threw pies at him remember! I was there for that!"
".....why you gotta storyjack me, man"