The original story that has been around more else less since the beginning is that with the wolves, dark angels, and ultramarines all breaking through the traps they had been set up for, he decided to gamble it all in a personal duel. He knew that he couldn't get into the palace before the remaining loyalists showed up and that his forces, spent from fighting a nasty siege would be caught between the remaining palace defenders and those three legions.
Also the loyalists on Terra are unaware of this (re-enforcements), it's only Horus' team who get to know this through their connection with their warp allies. So Horus lowering his shields is quite an attractive offer to the Emps to finish this war. Having said that I think he would still have gone to meet his "son" for a showdown rather then giving him a chance to escape (even in defeat) (
IMO).