I don't know about 'all of a sudden' but it makes sense that the sole would be sanded smooth by contact with abrasive terrain (sidewalks, for example).

I have used the product and others like it, and find it is pretty effective.

It's possible you have worn them a bit too smooth...or you may have stepped in something that made the rubber slippery.

I was at a fast food place two months ago, and there were slippery floor warning posts pushed to one side. They had washed the floor, and it was dry, but they had not rinsed it properly. It was still slippery when dry. My rubber soled sneakers remained slippery ALL the rest of that day. I was skidding all over the place.

I wear ballet flats most of the time and I've never had a problem like that with them. I'm wearing them right now.