An embolism can develop on the device itself and then travel through the patient bloodstream to the brain.
The hazardous situation in this case would be the embolism reaching the brain, and the hazard. Not sure why the embolism would develop in the device, but the hazard may be something like incorrect output or loss of function.
Also, the act of implanting the device means the interior walls of the vasculature may be damaged or there may be plaque in the vasculature that breaks loose from the vascular wall and is large enough to block an artery in the brain.
Does this only occur with implanting the device or is this a general possible problem with implantation?
With all of these hypothetical situations, there is no way to know exactly what happened to a particular patient.
If you don't know what happened, you cannot manage risk.