Reasons for decaying orbit for satellites and spacecrafts 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?
 A: The #1 cause of low-earth-orbit decay is atmospheric drag. There is just enough air up there to cause a tiny amount of drag, slowing the satellite down just like an aircraft without engines. End result: everything in LEO will return to the surface rather soon. 
Things that are higher up, like geosynchronous satellites or things in high inclination orbits that spend very little time at LEO altitudes will stay up longer - in many cases long enough that the de-orbit calculations become unrealistic.
A: Solar flares etc can heat the highest reaches of the upper atmosphere and make it expand outwards, thereby increasing the drag on satellites. Using an artificial heating via HAARP was explored as a way of altering the trajectory of incoming ballistic missiles across the pole. Allegedly without much effect (so the US govt claims)
A: If the satellite is big enough, or trails a long antenna / tether, there may be magnetically induced electrical effects as it traverses Earth's geomagnetic field. This has been suggested as a way to passively drop old satellites from orbit, no fuel required. Also, 'pumping' the system with solar power could raise the orbit. In theory, you could use the system as a battery, trade a slowly charged orbit for a burst of power.
Sadly, experiments with tethers often struggle; at worst, an angler's miscast may form a 'birds nest', but tangling hundreds of metres of tether around its host satellite is surely 'Game Over'.
