Can titanium rods from a craniocervical fusion ever "pop out" or the screws detatch themselves and eventually have to be removed surgery was six years ago head/neck was hyperextended a bit today severe numb/tingling/painful sensation in back of skull and

Yes. Though rare sometimes the screws can be worked out, or are dislodged. If you experience symptoms you can consult with your surgeon to evaluate your repair.
Yes. Metal fatigues , takes a long time, but can happen, usually a regualr xray will tell, but that doesn'tmean you have to have them removed.
It . It is unusual for the screws to break or pop out 6 years after surgery, but that said your symptoms are very concerning. I would recommend immediately contacting your surgeon about the severe pain, numbness and tingling for x-ray and follow-up. If you can not contact surgeon you should go to the emergency room to be checked out.