Likely as bad parenting, and rebellion against parents.
If she was in US, she probably would have become a radical anti-Israel leftist, who worships Chomsky.
If she was in 1930s Germany, she wold have joined the Nazis.
Since she was in Muslim-land of France, she became Muslim.
That there are now 2 French Jewish girls that have joined ISIS, is bad news.
However, we should bear in mind, hundreds of Israeli Jewish girls are converted to Islam, every year, inside Israel.