What causes hypertrophic obstructive cardiomyopathy?

Hypertrophic obstructive cardiomyopathy of the intraventricular septum narrows the outflow tract.
This, along with dynamic changes during systole, cause the higher left ventricular pressure to overcome the obstruction.
There are other types of hypertrophic obstructive cardiomyopathy that do not necessarily affect the septum or LV outflow.