Nambudiri Brahmins, popularly credited to have turned Tamil into Malayalam by adding Sanskrit words to it, were originally Tamils, according to this new paper. (Via Indo-Eurasian_Research Yahoo Group).
That makes sense to me if I agree with that Tulu Brahmin who claimed they were originally Malayalis who came along with Sankara. That also tells us that Malayalam didn't adopt Tulu Grantha script at a later time. It was already there in Malayalam region before its appearance in Tulu region.
update(22-Jul):If I think about it, I suppose this also accounts for Na, La, in Brahmin dialect of Tulu. Non-brahmin dialect of Tulu lacks these retroflexes.