These are people raised in a frum home who after some time, maybe while they were in high school, started shortening/lessening their clothing, questioning their parents, smoking, drinking, doing drugs, talking to/touching the opposite sex, removing daily routines (ie- washing hands, saying benching, kissing the mezuzah, getting up for davening, tefillin) which leads the neighbors to refer to this child with the catch phrase.
"You don't wear tzitzis one day, the next you'll be off the derech picking up shiksas in a bar on Shabbos!"