What causes blood or blood clots in your blood vessels to leak in brain ?

'Leakage' occurs when tiny blood vessels break or burst. Clots on the other hand occlude the vessels. But the end result of both is the loss of blood supply to the area and hence harm.

Blood clotting in brain vasculature is secondary to "hardening of arteries" associated with high lipids, diabetes, smoking, etc., resulting in stroke. Bleeding intracranially can be due to aneurysms, arteriovenous malformations, hypertension, trauma, anticoagulation, hematological clotting issues, etc.

A "blood clot" inside a small vessel in the brain often comes from the heart or a piece broken off from the vessel wall of carotid arteries--causing an "embolic stroke." A bleed or leaking blood vessel causes a "hemorrhagic stroke" because the vessel wall is diseased/weakened --"water pipe broke" Smoking, diabetes, high cholesterol/blood pressure, infection, aging etc..= causes for weakening.