What orbital do electrons fill

I understand according to Hund's rule that 2Px, 2Py, 2PZ, must be filled before any of the three gets a second electron. But what determines the next spot. Why can't the electron just as readily enter 2Py next or 2Pz? Is there a reason it has to go to 2Px?

Once the electrons are positioned in a subshell (for example "p"), they must initially fill each orbital with one electron because of "Hund's rule" stating that electrons must be "unpaired" unless they have to be paired. So once each "Px, Py, and Pz" is filled, the next electron does not have to go on Px (but we do it because of convention/trend). It can go on either of other two orbitals, Py or Pz BUT, the spin quantum number must be opposite from the previous electron occupying it (Thus, if the first three unpaired electrons in the P subshell are facing down/spin -1/2, the fourth electron can pair up with an electron on either orbital within the subshell but have to face up/spin +1/2, and vice vera, which is regulated by the "Pauli Exclusionary Principle").

**Thus, it doesn't matter which orbital (magnetic quantum number) the 4th (or 5th) electron chooses after all of them are initially filled with one electron each, under Hund's rule because, the orbitals within the subshell are of the same energy, thus it is not considered to be in an "excited" state because there was no "skip" in energy (which would be a violation of the "Aufbau Principle").

***Yes, Hund's rule only requires that the electrons be initially "unpaired" in each orbital (Px, and/or Py & Pz). There is no violation of the "Aufbau Principle" because, no change in energy was made (unless you didn't completely fill the "2s" subshell and moved on to the "2p" subshell, then that wouldn't be the ground state anymore because the 2p subshell is greater in energy than the 2s). Thus, in multi-electron atoms like Carbon, the energy within the 3 P orbitals within the P subshell are equal, thus the atom will still be in the ground state.