Stroke is often caused by a blood clot that has formed and blocked an artery leading to the brain. In this case, there was a second diagnosis of hypertension, which means that the dog might have had sustained high-pressure levels in its system for quite some time before having a stroke due to this long-term effect.
In many cases, however, stroke is not preceded by any previous noticeable symptoms. The event can happen at any time without warning, and it would be wise to take strokes very seriously as they are one of the major causes for broken hearts when a beloved animal has passed on without warning or concern from owners who thought that their pet "wasn't feeling well". Be sure you're giving your fur baby regular checkups...