The Incident – The losties cause themselves to be stranded.
The idea that the Losties are actually CAUSING the incident which causes them to crash has been thrown around, however I didn’t see any mention of Pierre Chang. I think the fact that in the orientation video he has no hand(or arm?), and that he says that there was “an incident”, would seem to indicate that this incident was THE original incident, as he has his hand crushed. Now of course anything is possible, after all if nothing can really be changed by them being there, then the same incident could have just happened under different circumstances. However if they CAN change what happens, then they cause their own plane to crash. This of course leads to a paradox, how can someone cause themselves to crash in the past, like, how could it have happened the first time? Perhaps, Jacob is the answer to how it got started. But it does seem like there is a timeloop problem going on.