A senior pilot will bid a desireable trip in a international category. He will then monitor all the LCA trips. He will find one he likes with a TOE on the trip. The day prior when he is sure the TOE is happening he calls up the junior pilot and swaps his better trip. He now becomes the senior pilot and stays home. The original senior pilot is now the junior pilot and has to fly a trip he thought he was going to be released from.
Additional fallout happens when the now ****ed off junior pilot calls in sick forcing the senior guy who did the last minute swap to now fly the trip. Sometimes they then call in sick. The company does not like this.
I posted a solution I thought was reasonable to stop this a few months ago. That would be releasing the senior pilot from the original PCS bid run first even if a more senior pilot swapped into the other FO slot after the initial bid award.