Malta *spoilers all Hobb books*

['thul]

That could also be it. Though it could be both.
Dragons do tend to have a certain attraction to the fluid skill that the fool got on its fingers. The touch of such fingers would probably attract any dragon.
Combine that with the fact that dragons want strong personalities in their potential Elderlings, and you get ample proof that Malta was just the right thing.

Tintaglia had long claimed Reyn. She had probably got a strong connection to his mind. When he became attracted to Malta, he would probably have formed an unconscious connection to her. Tintaglia would have sensed it, and perhaps utilized it, preparing to claim her as well. The main changes started after her hatching, but the exposure started long before that.
One thing these beings wonder about is whether Tintaglia ever gave her any blood/scale to hasten the transformation. Selden/reyn most likely got theirs upon the hatching, though not fully intentionally. It probably just happened.