My understanding is that any player with salary over 10k becomes a free agent after their team goes bot. Also, any player on the NT or U21 team becomes a free agent as well, so if you have a good 8.5k 19 year old player on a team about to go bot, national team coaches may add them to their roster to save that good prospect from retiring, if he's 20 and still has 8.5k salary, he isn't really a good prospect anymore.