In short...
In short, not a single subject, having any relevance to religion, was left without a discord of one type or the other. And this divergence, not unexpectedly, showed itself in exegesis of the Qur'an. Every group wanted to support his views and opinions from the Qur'an; and the exegesis had to serve this purpose. The people of tradition explained the Qur'an with the traditions ascribed to the companions and their disciples.
They went ahead so long as there was a tradition to lead them on, and stopped when they could not find any such tradition (provided the meaning was not self-evident). They thought it to be the only safe method, as Allah says: . . . and those who are firmly rooted in knowledge say:' "We believe in it, it is all from our Lord. . " (3 :7). But they were mistaken. Allah has not said in His Book that rational proof had no validity.
How could He say so when the authenticity of the Book itself depended on rational proof. On the other hand, He has never said that the words of the companions or their disciples had any value as religious proof. How could He say so when there were such glaring discrepancies in their opinions? In short, Allah has not called us to the sophistry which accepting and following contradictory opinions and views would entail.
He has called us, instead, to meditate on the Qur'anic verses in order to remove any apparent discrepancy in them. Allah has revealed the Qur'an as a guidance, and has made it a light and an explanation of everything. Why should a light, seek brightness from others' light? Why should a guidance be led by others' guidance? Why should "an explanation of everything" be explained by others' words? The theologians' lot was worse all the more.
They were divided into myriads of sects; and each group clung to the verse that seemed to support its belief and tried to explain away what was apparently against it. The seed of sectarian differences was sown in academic theories or, more often than not, in blind following and national or tribal prejudice; but it is not the place to describe it even briefly. However, such exegesis should be called adaptation, rather than explanation.
There are two ways of explaining a verse - One may say: "What does the Qur'an say?" Or one may say: "How can this verse be explained, so as to fit on my belief?" The difference between the two approaches is quite clear. The former forgets every pre-conceived idea and goes where the Qur'an leads him to.