Take the hammer for example. If you throw the hammer up into the
air, the hammer head part, which is the heaviest part of the whole
hammer, will always wants to fall first. However, if the hammer
head is positioned in the middle (which is weird 
) , the
whole hammer may want to fall straight, in a level manner, meaning
the whole body falls together, as compared with head first in the
first explanation.
Saw this explanation on Discovery channel of dunno what show,
cannot rmbr, while explaining the problems of rear engine cars.
This also applies to rear engined buses. If the bus speeds and
turns, the heavier rear part with the engine, may cause the back of
the bus to be "pushed" sideways, causing the bus to be out of
control. However, if the engine is positioned in the middle, the
weight is distributed evenly and the chances of the bus being
"pushed" out is lessened.
In other words, the heavier rear part of rear engined buses will
have a greater inertia at the rear when turning, as compared with
mid-engined buses.
