uhhhh :p may be earth quake?? :p (really may be)
You see id like to know how old your camera is, and did you drop it or crash it at some point of time?
why i am asking is because, if you are cent percent sure that your tripod is stable then,
the sensor may have become a bit shaky if in any of the above case...so no matter what shutter speed and stability you set, the photo might get blurred if the sensor inside your camera shakes....(even if the shake is like micro meter :p if the sensor is loosely held by the camera ) it may even shake due to tiny vibrations like mirror flip.
this is the only reason i could come up with as per your description. so i would prefer you taking your beloved camera to an authorized service center and giving it a full body check up :p :)
happy clicking :)