I thought the idea was that reality extrudes an error message when they meet, because the Source of Magic can't decide what should happen. And when this error message meets...whatever causes prophecies...we get Harry's sense of doom (which seems stronger after his first Patronus casting and then after his recent resolution). I admit that second part confuses me.

Hopefully - since the oldest prophecies we know of supposedly refer to "the end of the world and its magic" - they all result from Ohtori "It" Harry reaching back in time.