Pickled onion cider instead of pickled onions I have been making pickled onions for years with no problems.  I have recently bought pre peeled onions and find that in the pickle, these onions fizz in the jar.  

Why is this - the onions look fresh and healthy?
The situation you describe sounds like natural Lacto-fermentation.

There are many airborne yeast around us all the time. Every culture on earth (no pun intended) has a some tradition of fermentation for storage purposes. It would seem that you have just the right conditions for that to occur.

This can be a good thing. It creates a product with properties than many find healthful. A few resources are here:

Red Onions

More Red Onions

Same principle for white onions. If you submerged the onion goodness you have going on with a clean disc of some sort you can finish the fermentation and have a yummy condiment !