  Bohmian mechanics is a causal interpretation of quantum mechanics in which
particles describe trajectories guided by the wave function. The dynamics in
the vicinity of nodes of the wave function, usually called vortices, is regular
if they are at rest. However, vortices generically move during time evolution
of the system. We show that this movement is the origin of chaotic behavior of
quantum trajectories. As an example, our general result is illustrated
numerically in the two-dimensional isotropic harmonic oscillator.
