Pickled onion cider instead of pickled onions
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 !