You are saying this insert code is part of a shortcode handler? I've noticed several times that actions and filters often fire more often than one would expect. I've not observed this in shortcode handlers, but it's not a big stretch of imagination to add it into the actions and filters with unanticipated events category.
Sometimes I've been able to trace the cause, other times it's remained a mystery. Until I have lot's of spare time, it will have to be "just one of those things" for me.