It sounds like Instagram is testing a new "Save Draft" feature. 

Savers

Certain Instagrammers have reported seeing a dialogue box pop up offering to save their drafts, when hitting the Back button after composing a post.

Until now, backing out of a post would result in any content being lost.

Baby steps

While the update isn't massive, it would be a welcome addition for power 'Grammers and marketers managing multiple Instagram accounts.

Plus it might also help keep users on Instagram and away from Snapchat.