Answer:
Quantum Mechanically. They do not follow a trajectory or path in any normal sense. All the orbitals (not just the p) are statistical "clouds", the usual "pictures" shown are just a boundary enclosing the volume inside this cloud where it is most likely to find the electrons, there is a probability they will be outside this volume but still in the orbital. At one moment an electron can be at any arbitrary point in the orbital, then at the next it can be at any arbitrary point in the orbital (different or same point), it cannot be predicted.