Albo spadł na Wyspę na skutek wcześniejszej 'pomyłki systemowej'
May have crashed on the island after being transported through one of the network of Bermuda triangle-like areas known as the Vile Vortices. Inside the plane was a map in which the word SAHARA was prominently featured. One of the Vortices is located over the Sahara. This could explain why the plane is seemingly so far from Nigeria.
Judging from the map found on the Beechcraft in "Deus Ex Machina" the plane may have been headed towards Inedbirenne Airport in Algeria (24 17' 31" N, 9 27' 08" E).
May have been crashed on purpose by the other's as a way of preventing Locke from finding The Pearl station. They could have feared for its discovery when Boone and Locke stumbled upon the area, and pushed the unstable plane off its perch to cover the entrance.
Very unlikely: A simple rock could have the same effect.
If Mr. Eko's brother Yemi died while the plane was airborne, his spirit/ghost could have caused the crash in an effort to help Mr. Eko after having learned of future events. We can assume Yemi's spirit/ghost has some knowledge of the future. In the "?" episode, Charlotte Malkin tells Eko what Yemi told her while she was between places, including 'seeing him soon'.
It was placed on the island deliberately by someone or something.
It's part of Eko, Charlie, Locke, and Boone's destiny on the island.
The reason for Locke seeing the plane crash in the dream in the first place was the island's way of helping him find The Pearl, not the beechcraft. But it failed in its attempt, this is why it had to rely on the help of Eko to find it.
The Island did not want Locke to find The Pearl when he discovered with the Beechcraft with Boone, so it made his legs useless thus preventing him from climbing the cliff and seeing the ?.