He doesn't know why he first saw them,
i tried to put some very subtle and peculiar inconsistencies in the way the grammar is phrased here. you might think it should say 'suddenly i saw the mountains which i had never noticed becore, and i don't remember them being there.' but it's different to say ' i don't remember Why i first saw them.' basically i'm taking common phrases but turning them around just a little bit in ways that i haven't quite heard before, but they feel the same.