You used the word "why?" in there somewhere. My answer:

Most of the big splits were due to human quest for power, wealth, and glory.
Occasionally a split from a split was a reformation to get back to original Biblical Christianity.

You can generally tell which is which by whether the leaders of a split gained power, wealth, and glory from the sect they created.

The original apostles all suffered tremendously, gained no wealth or power and all but one died gruesome deaths for refusing to compromise their beliefs.

So it's easy. Simply ask, "Que Bono?" - Who benefits?

Christianity is defined by the original Hebrew, Greek, and Aramaic manuscripts of the inspired Scriptures. The further away from those we get, the further off the mark.

As Paul put it:

...I have worked much harder, been in prison more frequently, been flogged more severely, and been exposed to death again and again. Five times I received from the Jews the forty lashes minus one. Three times I was beaten with rods, once I was pelted with stones, three times I was shipwrecked, I spent a night and a day in the open sea, I have been constantly on the move. I have been in danger from rivers, in danger from bandits, in danger from my fellow Jews, in danger from Gentiles; in danger in the city, in danger in the country, in danger at sea; and in danger from false believers. I have labored and toiled and have often gone without sleep; I have known hunger and thirst and have often gone without food; I have been cold and naked. Besides everything else, I face daily the pressure of my concern for all the churches. - 2 Corinthians 11:23-28

Any leader of a sect who rose to a position of power, lived well, and granted themselves special privileges in such a way that there was a struggle by those who wished to succeed them can therefore immediately be eliminated and their teachings rejected.