Starting March 24th, SharePoint Online has been updated by enabling by default document versioning on Document library.
This impact all new document library created since then, as well as new site. The document versioning enabled is the Create major version
So be aware of this change has some impact in the storage consumed as well as to the user experience as draft version may not be viewable
More details https://support.microsoft.com/en-us/kb/3050531