mostly valsalva maneuvers: sneezing fit, coughing fit, lifting something heavy, constipation, etc. anything that would cause a temporary raise in blood pressure in your head.
but the 2 other big causes are:
1) undiagnosed high blood pressure
2) IDIOPATHIC (which means no one knows! or just bad luck)
if you have one isolated event, dont worry too much about it. it'll take a good 6 weeks to heal. if you have more than say 2 a year, i'd be thinking undiagnosed high blood pressure (unless you're already on super blood thinners like coumadin for heart disease...that could cause it as well)
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