By accident, of course. Is this a trick question?
Of course I don't know about these things, but it seems that a fixed wing plane would be fying higher that the buildings. The impact is at the middle of the building so it seems odd to me. Am I making sense?
Did you see the pics of the building? It's a VERY large apartment highrise building. If a plane was having maintenance problems, and was descending while over NYC because of that problem, there are HUNDREDS of buildings of that size and much taller in the area.