Pilots bid into bases on seniority when that base has a vacancy. They don't bump junior people from a type or a base that isn't allowed and has been made clear many times in the past. Our agreements protect everyone on the seniority list and long may it last.
If you feel someone has jumped seniority then let both Balpa and management know of the situation. I am unaware of the anomalies you talk of, but stand to be corrected.