I'm assuming you didn't read the link provided.
In short, a failed prophecy in Isaiah 19 declaring Assyria and Egypt would convert to Yahwism and a 'stone' monument built to Yahweh in Egypt, and the publication of coded Pyramid measurements that seemingly contained scientific facts only God would have known, was all to coincidental to Russell's mind.
His fascination was not unique, it was shared by other Adventists of the day.