Theories on What Happened with Amber and Matt

So Matt and Amber's wedding is on hold. Big surprise, or no? We're jogging our memories and it seems like this has happened before - Matt and Amber have certainly delayed their wedding in the past. And of course, on screen, all we see is these two being lovey-dovey. So what gives? Why would Amber and Matt put their wedding on hold? We've got three fan theories: