Before I get into this anymore just let's get some background info on what happens while the meteoroid starts to enter the Earth's atmosphere what happens when a body falls through the atmosphere. Meteoroids enter the atmosphere at speeds around 10-40 kps although some have been found to enter at 70 kps. As it enters the atmosphere of the Earth it collides with millions upon millions of air particles. As this happens the ourt layer of the meteorite starts to heat up due to friction caused form the collision of the body with the air particles. Most meteoroids tumble during their flight through the atmosphere however a few have been showen to maintain one orientation during its flight.
Iron meteorites are less subject to break up in flight due to their greater structural stability.
Now, as the body is falling the heated zone on it is thin so when the object reaches the ground the heat is quickly lost. This is the fusion crust. This crust is thinner on the leading face and on the forward parts of the side faces, but on the trailing surface a thinker layer of the fusion crust has accumulated due to the force of the oncoming air pushing the molten material towards the back of the meteor. These are formed because this surface is under a considerable ammount of air pressure. This is the same effect as when a large truck drives by you and you feel a big gust of wind.