It was likely caused by the transition from right-to-left to left-to-right writing

So basically, while Egyptian hieroglyphs didn't have a specific writing direction, boustrophedon was a very common system, where you'd start writing in one direction, and then switch direction on the next line (so the first letter of the second line is right under the last letter of the first line).

The Phonecian script, however, started to stick to right-to-left for writing. When the greeks first adapted the Phonecian alphabet for the Greek language, they wrote in right-to-left and boustrophedon. When writing boustrophedon, they would flip the letters to match the writing direction. When left to right started to gain popularity, the flipped letters were used, and left-to-right is now what is used in the Greek alphabet and its descendants.