#1) They come from a prosletyzing Protestant family and Orthodoxy makes them a stranger from their family so they leave it.
#2) The amount of Church attendance required. People want a lazy, minimalist religion where all they have to do is 'love Jesus' and go to Church once a week for an hour. But Orthodoxy is an entire lifestyle that requires constant fasting, attending Church all of the time, multiple times a week sometimes, and when you are at Church, you have to really dedicate yourself and participate opposed to just sitting down and hearing a sermon.
#3) They cannot find an Orthodox Church close enough to them, so they eventually just burn out from lack of guidance and start attending the local Protestant and/or Roman Catholic Church and eventually fall into those crowds.
#4) Too much uncertainty about what happens directly after we die. This honestly frightens me more than anything.
#5) They meet a mate who belongs to another religion and/or other form of Christianity and they become tempted to leave Orthodoxy to make their mate happy and/or make things simple for their children so that two parents don't have different beliefs.
#6) No one at their Church is nice to them. Let's be honest, those Evangelical/New Protestant Churches that we love to mock so much put us to shame when it comes to kindness and welcoming new comers. I'll admit, I have never had a problem with this at my Church (Possibly because I attend an OCA Church which is the most 'Americanized'), but I have heard of many negative experiences people have had with other Orthodox Churches both from fellows at my Church from people on this website. These happy, American, potentially convertsky fellows enter their local Orthodox Church expecting to be welcomed with open-arms by happy, jolly folks. But all they find is a dead place with a bunch of quiet, somewhat rude people staring at them negatively, talking about them in their native tongue, ignoring them and making them feel unwelcomed. People have spiritual needs, they need to see kindness and be welcomed, and if we as the Orthodox--the 'light of the world'--cannot satisfy those needs for them, then I am afraid we are in a really bad situation.