Say a pilot goes out on a medical LOA a fall where he hit his head hard.  

https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/aam/ame/guide/media/Head-Injury.pdf

He is out on a Medical LOA.   After a month or two, Company posts a temp position at his base for his absence.  Nobody internally takes it, because they don't want to transfer since it is posted temporary.   After two weeks, the company posts the job externally, and a new hire signs up for the job.  It takes the new hire almost two months to get through new hire classes and training in the aircraft.   We're 4 to 5 months into the 12 already.  

The new hire finally shows up and starts work 6 months after the pilot got his head injury.

In 6 months, the new hire will claim that coveted position becasue the pilot out on a medical cannot get his Class II back in 12 months!   The medical pilot, if he retursn will have to find a job elsewhere, move, and watch this new hire (15 years his junior maybe) own his old position!

But, lets say, the medical pilot finds out the FAA won't issue him a special issuance afterall, for some relapse.  Now, the new hire who's only been on the job 6 months has jumped the seniority list to claim a coveted location more senior pilots would have gladly taken if the job was posted not as a temp, but as a regular open position.   They missed out now on a chance to get their job, and see some new hire (15 years their junion maybe) own the job they really wanted awaiting it natural opening.

12 months doesn't protect the existing dues paying union pilot, neither the one out on a medical, nor the one who might want to bid the position normally (not chancing losing what they have on a dice roll whether the medical pilot returns).