What is the pathophysiology of hypotension?

Many. Hypotension or low blood pressure can happen for a variety of reasons. Acute illness (i.e. Sepsis), adrenal gland insufficiency, overmedication, heart failure are just a few of the many reasons for low blood pressure. Additional causes can be volume depletion (orthostasis), or a variety of neurocardiogenic causes such as pots or similar.
Hypotension. Hypotension implies a decreased peripheral vascular resistance, decreased intravascular volume and/or inadequate cardiac force generation in systole. Normal pressure has adequate amounts of all these to result in a normal pressure range. Decrease in one or all contributes to hypotension.