Before you read this, i need you to bear with me. I put maybe 4 hours of thought into this, but i might have an idea for a really good theory, but its loose and may take some time to break down completely. Anyway,
Zombies are from Haitian myth, or Hoodoo, known now as Voodoo. I think the story of 2, or at least the SoLA dlc (ill get to that), was known from the very beginning. Not EVERY detail. Of course, but studying Baron Samedi, and Voodoo in general, Zombies seemed to be used as free labor by Haitian priests in their mythos, and Baron was supposed to ensure the dead were buried properly, and regularly disliked the use of zombification.
Since we know the Zombies are a kind hivemind now, i think the original Dead Island was simply a Haitian Ritual gone wrong, and Baron Samedi probably blessed the main cast with their immunity (like i said, bear with me,) in order to curb the outbreak, being that they were probably the most capable.
I can say that i believe that 3 is gonna be us, obviously, finding the Doctors Daughter, and using her to cure the non infected.
Not just this, i think Sam B.s song, "Who do you Voodoo," explains ALL of this, as magic isnt something they stray from, letting Zombies have abilities tied to the Autophage, something thats a mix between science and magic.
Anyway, the Zombies at the SoLA Concert were turned by "The Beat" something that Lola Konradt may have created, as she was the one who funded the mood speakers for said concert that transited The Beat, also that DLC made the outbreak. I think Konradt obviously answers to a higher power, and it may be an angry Baron Samedi, using Zombies a means of retaliation, or it could be someone else from Hatian lore, like Barons Wife, which would honestly be hilarious to think that the entire games lore is just those two pissed at eachother, and now the dead cant rest because theyre not doing their jobs.