When you start working on a project, loose motivation for it (even though in many cases you don't like admitting loosing motivation for it) so you just start working on something completely different and convince yourself to come back to your other projects in a later future (even though most times this won't happen and you'll just end up making the project die).
Mike: Wasn't Mark writing that book about flowers?
John: Oh no, he project++ed it a few days ago to work on his other book about felines.
David: Sean is a hardcore project++er, he's got like 40 unfinished projects.