The first time it fell off was probably due to bike shop incompetence in incorrectly torquing the axle nut. This happens a lot when lazy shop mechanics assemble bikes without using the torque wrench. The second time was probably due to damage the crank sustained when it fell off the first time.

In any case, you probably can't get anywhere near the necessary torque using just an allen wrench. It is important that a torque wrench is used.