That was likely a bit of retconing since he wanted the hobbit to fit into the lord of the rings. Like the way he took out some chapters and rewrite them to fit in with the lord of the rings.
It could be said that the ring left Gollum since it was trying to get back to it's master, as I think has been mentioned somewhere, forgot where. It could be it was hoping some orc would find it, but instead Bilbo did by chance. If one wants to stretch things a bit one could say, though it would be a real stretch, that some maia, or valar or Illuvator himself subtly did something so Bilbo was knocked off of the dwarf that was holding him so the ring could be found by Bilbo and not an orc, but that would be a real stretch.
Maybe part of the reason it left Isuldur, hoping to be found by the orcs who were nearby, but since Isuldur was in a river or lake, forgot which, it fell to the bottom where it could not be found by them. & it tried to leave Bilbo & think when he was near one of the entrances to the orc/goblin lair, but since his hand was in his pocket it just fell off of his finger & stayed in his pocket and failed again.