The Cause of the Separation of The Shi'ite Minority from the Sunni Majority ?
Thus the true friends and faithfull Followers of Ali Ibin Abe Taleb(as) stuck firmly to their belief that after the death of Prophet Mohammad Al-Mustafa(pbuh&hf) the Caliphate and Religious Authority (Marja'iyat Al-Ilmi) hence only belonged to Ali.
This belief came from their consideration of Ali's position and station in relation to The Holy Prophet Mohammad Al-Mustafa(pbuh&hf). His relation to be the chosen from among the all companions, as well as his relationship towards the Muslims in general.

It was only the events that occurred during the few last days of The Prophet's(pbuh&hf) final illness that indicated that there was opposition to their view. Contrary to their expectation, at the very moment when The Prophet(pbuh&hf) died and His Holy dead body lay still unburied.

While The Prophet's(pbuh&hf) Household and a few of his faithfull Companions were pre-occupied with providing for The Prophet's(pbuh&hf) burial and funeral service.

That the friends and followers of Ali received news of the activity of another group who had gone to The Prophet's(pbuh&hf) Mosque where the whole community of the Muslims was gathered.

The group supposingly were consoling each other and the gathered Muslims before them, faced with this sudden loss of its Leader.

This group, which was later to form the majority, set forth in great haste to select a Calipha for the Muslims with the socalled aim of ensuring the welfare of the community and solving its immediate problems.

They did this without first consulting the "Holy Household" of The Prophet(pbuh&hf), or even his relatives or many of his close and faithfull friends, who were busy with the funeral, and without providing them with the least information.

Thus Ali and his companions were presented with an afait accompli.

When Ali and his friends such as:

Abbass
Zubayr
Salman
Abu Dharr
Miqdad
Ammar
Were finished with the buriyal of the body of the Prophet(pbuh&hf) became aware of the proceedings by which the Calipha had been selected.
They protested against the act of choosing the caliph by consultation or election, and also against those who were responsible for carrying it out.

They even presented their own proofs and arguments, but the answer they received was that the welfare of the Muslims was at stake and solution lay in what had been done so unjustly.

It was this protest and the criticism
Which separated the majority from the minority that were following Ali(as) and made his courageous and faithfull followers known to the existing opposition the Sunnah as,

The 'Shia'at Ali Ibin Abe Taleb(as)
meaning

The Partisan's of Ali Ibin Abe Taleb(as).

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The caliphate of the time was very anxious to guard against this appellation being given to the Shi'ite minority and thus to have the Muslim society divided into sections comprised of a majority and a minority.
The supporters of Abu Bakr considered the Caliphate of Islam to be a matter of the consensus of the community and called those who objected,

"The opponents of their allegiance."

They claimed that the Shia stood, therefore, opposed to Muslim society as a whole.

Sometimes the Shia were given other pejorative and most degrading names.

Shiasim was then condemned from the first moment because of the political situation of the time and thus it could not accomplish anything through mere political protest.
Imam Ali Ibin Abe Taleb(as), in order to safeguard the well-being of Islam and of the Muslims, and also because of lack of sufficient political and military power, did not endeavor to begin an uprising against the existing political order, which would have been of a bloody nature.

An uprising then would have destroyed and dismantled all that The Prophet Mohammad(pbuh&hf) had painfully accomplished, and the danger of the newly established religon of Islam to disappear as quickly as it appeared.

So, instead Ali Ibin Abe Taleb chose to be patient and withdrew to work qiuetly by guiding Islam behind the scences.

Yet those who protested against the established caliphate refused to surrender to the majority in certain questions of faith and continued to hold, quite rightly, that the succession to The Prophet(pbuh&hf) and the religious authority belonged by Divine Right only to Ali.

They therefore believed that all spiritual and religious matters then should only be referred to him and subsequently invited the people to become his Shia or Followers.

Taken from "Islam : the living religioun " web site.