A superb question Sir!
From my own studies into Catharism and the theology of Marcionism I have found that the Cathars came to identify the Old Testament God as an evil force in contrast to the Good God of Jesus in the New Testament due to their belief that the world and all matter was corrupt. This belief in "Two principles" can be considered radical dualism in the sense that they believed:
- That there is one realm of good/the soul/the True God
- & another realm that is evil/all physical matter/the evil God
The Cathar's beloved book "two principles" outlines this theology in a very convincing manner.
It is possible that (similar to Marcion of Sinopes theology years before Catharism) they considered the apparent contradictions between the loving God of the New testament and what they read to be the vengeful god of the Old testament to be two principle powers in opposition to one another; thus concluding, like Marcion before them, that the Old testament God is the evil principle who brought about this world along with all its pain and confusion.
I have questioned how the Cathers would have discerned which texts refer to the words of which principle power? (which I have asked here:If the premise for Marcion's Two Principles (Dualism) is correct; which words in the Bible belong to the Demiurge?) Because, like you say, Satan is referred to in Job as discussing a plan with God.
During my study into the Cathars, I felt that they could on the outset justify their theology as it is a rather neat package in solving the problem of evil which all generations and denominations have strived for; the Cathars saw themselves as "Sparks of Divine in Flesh" (Romans 7:23;8:5 & Job 5:7); they no doubt yearned to be away from the flesh of this evil world (Philippians 1:21).
I also questioned the actual Cathar premise that how could there be two principle powers? One evil and one good? The only way I can imagine it (within human limitations) is that the good principle is infinitely more powerful than the evil; however He is also infinitely merciful towards the evil (love thy enemies and all that) therefore the two can exist indefinitely in this sort of theology. How the two came about would be the Cathar's theological mystery.
With regards the question: What was the explanation for why Catharism identified the Old Testament God as Satan? The answer is found in the Cathar's book of Two principles (Part IV. A Compendium for the Instruction of Beginners, section 7) which states the following about the evil principle/god and Satan/Devil:
 On the Evil Principle. For this reason, in the opinion of the wise it is firmly to be believed that there is another principle, one of evil, who is mighty in iniquity, from whom the power of Satan and of darkness and all other powers which are inimical to the true Lord God are exclusively and essentially derived, as was demonstrated above and will appear below, God willing.
Therefore it is evident from the above Cathar text that they saw Satan as a being who's power derived from the evil principle; therefore not the evil principle himself, but most certainly his agent...in contrast to Christ who is the True Lord's agent.
I hope this touches on answering your question.
All the best.