Some big planes full of fuel hit some buildings at high speed.

There was physical damage and fire.

This weakened the area of the strike, and the weight of the floors above was unable to be sustained.

So the floors above the strike area fell down.

The floors below the strike zone could not handle the impact, so they too collapsed, continuing all the way down.

It's pretty simple.

You need to invent a mythical bomb to explain the collapse?

You may as well say a magic pixie did it.