How this would defeat seniority would be...
Pilot A is senior and is based in DAY. Pilot B is junior, and is a friend of Pilot A, and wants to be based in CLT.
Since pilot B is junior, he can not hold CLT. But, Pilot A, a great friend, bids for CLT while pilot B holds DAY. Then, once the award becomes effective, they swap bases. This would allow the junior pilot to be based where he wants, even though he can not hold it.
There is an annual system wide bid that allows for transfers across all bases, but it is pretty rare not to have any movement in/out of a base for more than a month. The bi-weekly awards that come out have movement in every base just about every month.