The angle is not caused by forces, however, when given a magnetic moment that makes an angle with the B-field there will be an associated orientation potential energy [tex]- \vec{\mu}. \vec{B}[/tex]

This energy arises from the fact that a force (ie the Lorentzforce) will act on the moving charged particles. Also, a torque will be exerted on this L-vector because of this very reason. It is this torque that is directly responsable for the precession of L because a torque is equal to a change in L with respect to time :

http://hyperphysics.phy-astr.gsu.edu...magmom.html#c2
marlon