limitation of a specific essential nutrient, accumulation of toxic by-products, presence of stress factors such as changes in pH, temperature, osmolarity, etc.
The stationary phase of bacterial growth happens when the rapid logarithmic grow plateaus off and becomes level. At this point, the number of bacteria being created is equal to the number of bacteria dying, so there is no net change in bacteria numbers. i think that's it. I hope this helps!!:)