I also associate succumb with die for some reason, although I'm not sure why. Maybe in the newspapers if they say 'x succumbed to their injuries' it would imply that?
If you succumb to cancer or another often fatal disease - "fail to resist" it -- that clearly has to mean death. If you succumb to a cold -- failing to resist it just means you are stuck in bed with a bad cold. So I think it's probably because you are used to hearing it used with more serious illnesses, and in your mind a seizure is closer to cancer than a cold.