Although I've found comments from quite a few spinal fusion patients talking about having broken screws, I haven't been able to find any official medical explanation for why this happens.  The closest I've come is one patient who said her doctor told her that the reason the screws may have broken is because bone has some give but screws have none and the bones may have shifted just enough to snap the screws.

I can certainly understand why you would be afraid of having another surgery.  Don't hesitate to express your fears and concerns to your doctor.  You didn't say what happened when you almost died after one of your surgeries, but hopefully if you have to have another surgery, precautions can be taken to prevent something similar from happening again. 

Good luck to you.