There can be plenty of reasons why. It could be based on your appearance. Do you look like you wouldn't do well elsewhere? They could be filling a need.

I was surprised nobody else with more seniority wanted to transfer to where I am now.

Odds are, they did. The thing about management is they'll do whatever they want and it's up to the union members to complain. If they can avoid filing an injury report by placing someone hurt on the job somewhere else in the warehouse, they'll do it.

So my question is was I brought in just to fill in for those on vacation, and i'll be canned before the 30 days?

You wouldn't have been hired to fill spots vacated by vacations. That's not how UPS operates. They'll just make do with less. This is no indication that you'll be let go before you make union.