What causes speed wobble that's not tire balance/alignment?

Not the most likely cause, but you could have a separated tire. Some people think this problem would be easily seen. Not always the case.

 

If tires and all other parts are fine check to see how many rocks are stuck in you're tread or mud on the back side of a wheel. Those two problems plague me on a regular basis. Either one will cause a wobble with a perfectly balanced tire. Or, if you have the tires balanced once all the rocks are thrown out or the mud is washed of you'll get a wobble/vibration.