As per newton's law, unless a force is applies, moving objects will continue to move in a straight line. A satellite which speeds at around 17.5k mph would be travelling in a straight line considering the speed and the curvature of the earth (5feet dip for every 8km).
If that is the case, how can the orbit decay, just as happened to 'Venus Express'? Was there any reason of space weather where particles from solar flare disturbes the speed of the spacecraft and misaligned from its orbit? If so, what are the other reasons?