The official fall of Horus, as I understand it (given that I've not read Abnett's new series, and utterly detest the Sabretooth fluff) is as follows.
1) Horus falls ill.
2) He is recuperated in a warrior-lodge on Davin, which he was in the process of introducing into the Imperium. Nothing that unusual, Horus was famed for "going native" in order to bring worlds into the Imperial fold.
3) He starts introducing warrior-lodges into the Sons of Horus as part of their combat doctrine.
4) He turned to Chaos.
The speculation is that the warrior-lodge he found was a Chaos cult, and they converted him. How, I'm not sure. It's also speculated that given the Primarchs' brilliant immune system, that the illness he caught had to be Chaos-borne as well.