Why do some baalei teshuva go "off the derech"? There are of course many answers to this question.
One reason is the desire to jump into mitzvah observance too quickly, before a solid base of faith is built. Rav Tzvi Yehuda Kook zt"l used to say that faith is something that must be learned, just like one learns Chumash or Gemara. That is why the students in Merkaz HaRav and its related instutions spend time learning books like the Kuzari and the writings of the Maharal.
The rush to observe mitzvot before developing a solid faith stands in contradiction to the Mishnah (Berachot 2:2): "Why does "Shma" precede "Vehaya Im Shamoa"? In order that one should take upon himself the yoke of the kingdom of heaven first, and afterwards take upon himself the yoke of mitzvot.
Without a strong faith, it is easy to go "off the derech."
For more on this see Torat Eretz Yisrael by Rabbi David Samson pages 15 - 17.