Feral cats are essentially domestic cats turned wild. Domestic cats have developed the ability to live in groups.
The difference between the more natural outdoor environment and the artificial household environment plays a part in the creation of stress in multi-cat households.
Feral Cat Environment
In the feral cat, farm environment, groups of cats are usually made up of related cats that get on as a group. The group has evolved naturally. The food source dictates the size of the group. Individual cats in the group do their own hunting. These factors result in no or less competition for resources, which in turn restricts the need for a hierarchical system (dominant and submissive cats) and leads to reduced stress.
Domestic Cat Environment
In the domestic environment where there is a multi-cat household the cats in the group are less likely to be related. Normally, the cats' caretaker would have brought them into the group. The introduced cats are sometimes adults too. The existing cats would not regard these new cats are part of the same social group. The cats are living close together and there may be competition for litter resources and food. This is a recipe for stress born out of threats to territory and competition. Hierarchical systems have to develop to help avoid conflict. There is the potential for stress.