Ele se tornou ativo uma vez que ele tinha Quirrell.

O Lorde das Trevas só se tornou ativo novamente quando Quirrell cruzou seu caminho, dando a ele a oportunidade que precisava para voltar.

“Then … four years ago … the means for my return seemed assured. A wizard – young, foolish and gullible – wandered across my path in the forest I had made my home. Oh, he seemed the very chance I had been dreaming of … for he was a teacher at Dumbledore’s school … he was easy to bend to my will … he brought me back to this country, and after a while, I took possession of his body, to supervise him closely as he carried out my orders. But my plan failed. I did not manage to steal the Philosopher’s Stone. I was not to be assured immortal life. I was thwarted … thwarted, once again, by Harry Potter …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

Antes disso, o Lorde das Trevas estava esperando por um dos Comensais da Morte para encontrá-lo ou por alguma outra chance de retornar.

“I remember only forcing myself, sleeplessly, endlessly, second by second, to exist … I settled in a faraway place, in a forest, and I waited … surely, one of my faithful Death Eaters would try and find me … one of them would come and perform the magic I could not, to restore me to a body … but I waited in vain …’

The shiver ran once more around the circle of listening Death Eaters. Voldemort let the silence spiral horribly before continuing. ‘Only one power remained to me. I could possess the bodies of others. But I dared not go where other humans were plentiful, for I knew that the Aurors were still abroad and searching for me. I sometimes inhabited animals – snakes, of course, being my preference – but I was little better off inside them than as pure spirit, for their bodies were ill-adapted to perform magic … and my possession of them shortened their lives; none of them lasted long …”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

É apenas uma coincidência que Quirrell cruzou seu caminho no mesmo ano em que Harry estava começando em Hogwarts - sua escolha de voltar naquele tempo foi baseada apenas em ter Quirrell e não teve nada a ver com Harry.