Sometimes when a player sitting out or when he is disconnected and blinding out he just folds his last hand when he's all in, sometimes there is an actual showdown and the player sitting out can survive by winning the hand. I think this can have a major impact on what the optimal choice is in certain spots, but I have no idea when which is used.

Anyone has any idea?

i think when he has exactly one bb or sb or ante left he seees a showdown

No I had a situation this morning where a player had 1 ante + around 10% of the SB and he got a showdown.

Maybe it has something to do with sitting out due to disconnection (after a while you are seated out) and seated out due to inactivity?

A player who is sitting out cannot bet, they can only check.. or fold if a bet/raise comes their way. There used to be a rule called "All In Protection" which sets a sitting out player by default to "All In" if they disconnect... but that was mainly from the dial up days and isn't used much any more, saying that some tournaments still allow it.

Pretty sure as long as someone puts in a raise they fold. Ex. If they have <1sb and it gets walked to the sb and they are the BB they'll be AI. If the button shoves in the same situation however they'll auto fold. I'm 80% sure if the button calls the BB gets folded also.