By the end of turn 56 every city on the continent (and globe) was following my religion (Harappan Polytheism) to about 100%, no other religion was existent by then.

By the beginning of turn 57 the Egyptians have build the temple of Artemis (nothing else changed) and took over everything:

The religion is still called Harappan Polytheism and basically everyone follows it, but the religions leader is not me anymore, but the Egyptians? You can also see that the followers-bar to the lower left is now locked. WTF just happened?

Here are the saves: 

Turn 56: quick-save.ctr

Turn 57: auto-save 435.ctr

Being Religion Leader isn't just about founding it, the Leader is chosen among all the Empires following it and it's one with the most Holy Sites. So yeah, Egyptians building Temple of Artemis is precisely what triggered it.

EDIT: quote from Encyclopedia:

"Religious Leader

the Empire who dictates a Religion’s Tenets when multiple Empires share the same State Religion, determined by the Empire who possesses the most Holy Sites for that Religion."