I can see where socks is heading with that question, and I agree. HV P/O is a position that is earned through union seniority. Once a loader has enough seniority they can become a pick off. So what we seem to have is a supervisor that isn't doing something right from what you've told us:
It is possible that the loaders do have more seniority than you, and slowed their work pace down to screw both you and the supervisor since in this particular case you should not be the one picking off. The supervisor will probably also end up with a grievance if this is the case and it continues.
Either that or he is violating the contract between the International Brotherhood of Teamsters, and United Parcel Service by sending lower seniority employees home before giving you the option to leave or stay and help wrap up the loads. Loaders always have less seniority than a P/O.
I would just talk with the other employees on your belt, and find out how long they have been working at UPS. Make sure your suppose to be up there because if these other employees have more seniority than you, they won't do you any favors to help get you room up in the chutes.
Now if your supervisor is letting lower seniors leave before you, I would let him know that you are aware of the situation and that you would like to be asked to go or stay in the order of your seniority. This is something he has to do.
I'm sure socks has some great screw all union members reply, and I actually find some of them entertaining even though we all know that in reality people will only follow a management person out of intimidation for so long before they will lay down on you. It takes a good balance of firm and fair. In your case, your supervisor needs to start being fair to his employees and earn some respect before he runs the belt into the ground.
However, you should take pride in being a hard worker. UPS sure could use more of people like you at the P/T level. Socks is right in his previous post. Don't let stuff get under your skin in this place. It will eat you alive. Best of luck.
I know for a fact I have more senority than everyone else on my belt besides the main pcik off whos been up top for 5 years. The only 2 ppl on my belt who are close to me in senority is by 3 weeks, and thats me being ahead by 3 weeks. Out of the other 5 people besides those other 2 behind me by 3 weeks...3 of them were hired last peak season, and the last 2 were hired 2 weeks ago...I have been working for UPS since April of 08, which gives me senority...I can insure you I am supose to be up top. Soon as the other guy quit, they came to me about the position.