What causes hypotension when someone has malaria?

Many things. Malaria is a viral illness and cause fevers, during fevers there is vasodilitation which causes hypotension. Also as the virus attacks the body and the body attacks the virus many free radicals and chemical reactions take place many of which cause the body's blood vessels to dilate and cause hypotension.
Several causes. Excessive sweating, lack of drinking and fever all lead to dehydration and low blood pressure. Falciparum infection, the most severe form, also causes low platelets and disseminated intravascular coagulation (dic) with bleeding, resulting in low blood pressure and shock.