Iliad Book 20- Poseidon Intervenes

I find it surprising that Poseidon has a sudden sympathy for the Trojans at this point and saves Aineias from being killed by Achilleus. It seems to me that up until now Poseidon has been on the side of the Acheans for the most part. I wonder why this sudden shift? All he really says is that Aineias gave gifts to the gods and that he’s really done nothing wrong, so why let him be killed. That doesn’t seem like a very motivated reason to me for Poseidon to save him. But he does think Zues will be particularly angry if Aineias dies. Because “It is destined that he shall be the survivor, that the generation of Dardanos shall not die, without seed obliterated, since Dardanos was dearest to Kronides of all his sons that have been born to him from mortal women” (line 302). It goes on to say how Zeus cursed the Priam family, and that the Aineias family shall rule for the Trojans in the future. Is Little Brother just following the greater cause?