I understand that there are many differences in customs between Ashkenazi and Sefardi Jews, since they follow the rulings of the sages from their own communities. The differences in pronunciation (havara) can be put down to the influence of the local languages, and even the subtle differences in the prayers are according to what their Rabbis decided should be in the prayer book.
The one thing that stumps me, though, is how it came to be that there is a difference in how to put on tefillin. I mean, the day after Moshe taught everyone how to put on tefillin, presumably he taught everyone the same way. And if there were differences in custom, surely it would have been divided by tribe.
So the "day before" the Ashkenazi/Sefardi split happened, you would either have had a uniform custom of wearing tefillin (on the most basic level, winding inwards or winding outwards), else you'd have a generally random sprinkling of different customs, depending on which tribe everyone had come from.
Fast forward to today, where all Sefardim wind their tefillin outwards, and most Ashkenazim wind inwards (excluding minhag "Sfard", which winds outwards, but has a simple minhag for how to wind around the fingers, as opposed to the more intricate winding used by Edot Mizrach). How did that happen? Assuming the Sefardim kept their minhag the same, and the Ashkenazim changed the minhag, on exactly what day (and why?) did some Ashkenazi posek (who?) get up and announce that as of now, we're all going to put on our tefillin differently than we did yesterday?