I've always assumed that the last book would need to have so much happening that by the end of the second book all the loose ends need to be tied up so we can see the Battle of Caemlyn and then the later, final fight, probably with Rand dying between the two and being taken to Tar Valon on a boat for the Healing of Death by Nynaeve, which will require the sacrifice of Logain to allow Rand to be reborn.

Of course all that stuff is just my hypothesis, and it's based more on Arthurian legend than anything else.