I have a tandem axel with one trailer tire in particular that has almost no tread on the tire. The wear is mostly on the outside side of the tire, but the tire in general is literally almost worn out....while the rest of the tires are perfectly fine. Anyone know why this might be happening?

Bent axle or loose wheel bearings. If it’s a torsion axle could be worn or loose.

I agree with this. The first thing I would do is jack up that side of the trailer so both of the tires are off of the ground and take a look at both tires. Also grab each tire and check for slack like tmd11111 said, then spin them and look for wobble. You may have a bent rim.
Good luck on finding out what is wrong.

Thanks fellas

Bent axle or bent wheel.