If you use Dropbox Smart Sync, when you browse files in Dropbox folders to find an open/save location, Adobe softwares (AI, PS, Acrobat...) download automatically all the files in folders you are browsing !

It's said in the Adobe blog that Adobe dev team made an improvement in the march 2018 release (https://theblog.adobe.com/introducing-march-release-illustrator-cc/).
But we still have the issue on several macbooks at work. It's really boring.