What causes logistic growth?
I think it depends on what you mean by "causes".
If the rate of growth is proportional to only the current amount, then the growth is exponential. Exponential growth is unbounded.
In many cases, growth is better modeled by assuming that there is a maximum amount and that rate of growth does not depend solely on current amount, but depends jointly on current amount and "remaining space".
The mathematical explanation is that logistic growth occurs when the rate of growth is jointly proportional to the current amount and the difference between the maximum and the current amount.
If you are looking for a physical, biological, economic or some other "cause" for logistic growth, you'll have to ask people in those fields.