There are two senses in which people use the word “cult.” In the first sense, it describes abnormal, deviant religious groups. In the second, it can mean any religious group at all, but it’s most commonly used in this sense to describe a small group. The first sense is normative, in that there’s a value judgment being made that is based on what most people do, and the second is simply descriptive.
As such, “cult” probably isn’t the best word to use here. We want a non-normative way to discuss the negative, harmful things that we associate with small religious groups.
So what happens to make small religious groups harmful and dangerous? Well, in a word, politicking. The rot can be at the very top, with the charismatic founder of the group, or nasty behavior from the bottom can be passively tolerated. Both cases are a lack of proper leadership. Leadership is how the human power-grubbing instinct is curbed. Without it, there is a power vacuum and so individual members are going to step up to provide it.
These dynamics can happen in any small, insular group, it doesn’t have to be religiously focused. Leaders emerge in any group, and these leaders can sleep at the wheel and then the less-capable and less-virtuous can see an opportunity to co-opt the power of the group for themselves.
The religious, spiritual nature of the group really has nothing to do with it. It just so happens that awful people often find religion an easy way to acquire power.