'Ho-How is that even possible? I am immortal, I can't die. So I am getting death premonition? This does not make sense at all.'
Narada took step back in utter primal fear that in each being inherently has, Death, the fear of dying took over his action.
But on top of that the fear of the unknown occupied his mind, He is a big devotee of Lord Vishnu because of which he got the protection from death, He can't doe as the sage of gods.
And yet here he sensing that he will die if he dare to approach the this child, How can that be possible? He never felt this in front of any Asura he met and faced before.
Even someone like Ravana who captured Lord Shani the mighty god whom all gods are terrified of did not make him feel this neither did the Meghnath the son of Ravana and one who defeated Indra getting the title of Indrajit (The one who conquered Indra) gave him this feeling.