No, he had written himself into a corner at the end of the "Dance of Dragons", and then waited for the tv show screenwriters to help him figure out how to proceed from there - that's the most probable explanation I can come up with (hinted by some statements from his interviews).
I think HBO just made it way easier. Once I heard the series would be a show, I knew we'd never get any more books because the lull between A Feast for Crows and Dance with Dragons was something like 5 years. At that rate, he'd need another 15 to be done while gallivanting on other fantasy titles/collections/anthologies/wtf and endorsing everything under the sun once the first three books gave him clout.
I'm definitely a little bitter about it, but I've accepted it. Parris needs the royalties.
/salt: Somehow, within that time, Robin Hobb managed to spit out a trilogy seemingly once a quarter. Thankfully, Steven Erikson came along and wrote The Malazan Book of the Fallen, which, if you like books, honestly puts ASoIaF to shame on numerous levels (and has the benefit of being done).
