Brief Answer:
Unlikely

Detailed Answer:
I read your question carefully and I am sorry about the distress you seem to be in. I also read your past queries mentioning the MRI report.

I would have to agree with my colleagues in saying that episode wasn't what caused the patients stroke. That MRI result clearly indicates there were prior changes in the brain due to ischemia, the fact that they were multiple shows there was widespread damage to her blood vessels either from atherosclerotic changes or due to repeated emboli coming from the heart.

Stroke rarely comes out of the blue, there is some underlying factor underneath causing it like heart arhythmia, atherosclerosis due to high blood pressure, diabetes, high cholesterol, smoking etc or simply age. I am sure since you seem to have been researching you will have found that out. Otherwise if a simple startle or movement caused it, it would happen at any age, wouldn't be a disease of mainly elderly, would it?
Yes it may be that a startle precipitates it at a certain moment. That might be as stress hormones causing vasoconstriction or altering heart rate might be released. But that would happen only because that person already had narrowed vessels by atherosclerosis or had an abnormal heart rhythm, it was bound to happen sooner or later, with or without the startle. In this patients case I still do not think it was the case, her urinating and change in behavior might be themselves the manifestation of stroke, so when she was startled it had already happened.

I hope to have been understandable and alleviated your sense of guilt. I remain at your disposal for further questions.

---