Organic increased up from 20,000 to 25,000 at the same time paid search were stopped. I think there should be no dependency between organic and paid search, but could there be any meaninful dependency between these? e.g. Bad keyword set?
That's not really that surprising, and suggests that you have some decent organic rankings.
Imagine users are searching for a product you sell, and they see your ad listing before your organic listing; that's what they'll click first. If there's no ad shown, but your organic listing is the first relevant link, then many users will still click that.