Over at Fearsome Pirate, we read a reply to the criticism that the Protestants are to be faulted for the proliferation of these sects, specially Christian sects. The Pirate's thesis is that religious freedom makes such a phenomenon happen. I got nothing much to add to his great insight.
I will only add $0.02, to why is it that most of the time the sects that sprout out come from the non-Catholic "christian stream", meaning they invariably promote the idea of Scripture Alone or sola scriptura. OK, to what authority will one appeal to when a group is digressing from another? Of course it will be to the group's sacred text. This is not the fault of sola scriptura, for sola scriptura as a principle is inherent to every religion that has a sacred text. For example, the Muslims have two major streams each has its own take on the Qu'ran. The Jewish people have several sects or denominations in them all having their take on the Tanakh- the Orthodox, Conservative and the Reformed movements. Each nevertheless appeals to how they interpret their own sacred text. The Buddhists have their variations too on the teaching of Budha. Having various opinions is not the fault of the sola scriptura, it is a phenomenon that comes with the turf when a group has a revered text.