In general terms, this phrase refers to a relapse when it comes to leaving behind a habit that is harmful.

In GTD it is used when you stop doing some of the core habits (capturing, clarifying, organizing, reflecting, engaging), so that disorganization and chaos return again.

It is very common to fall off the wagon from time to time before you get everything working fine. New habits are hard to establish, so don’t get discouraged when this happens.