Its a combination between the words instagram and hashtag. It refers to a person that is obsessed with hashtags and writes all their comments with hashtags.
person 1: "hey have you seen the comments in Lisa's pictures, they are all hashtags"
Someone who tries to make everything inflammatory by encouraging escalation of actions, while pretending to be concerned about de-escalation, resolution, compromise, and intervention, or anything else that makes him/her look like a good person among bad people.
The girl's family knew she had always been an instigator, the kind of person who said fuck you to neighbors and flipped them off by coming in their town like she owned it, with the intervention programs she supported to cover her own actions, or involvement in the questionable actions of others, and make her look like a better/stronger person than she actually was.
The instigator would rather fuck with somebody else's life or talk shit about it than tend to their own (if there was one to tend to, if it wasn't a facade of a life to show everyone).