the closer to center your object ball is, the more throw you will exert on it.
ever see an object ball HOP when struck with a cue ball rolling at table speed? The cue ball is trying to THROW the object ball down, into the table. Since it cant go through it, it rebounds and produces this seemingly unexplainable hop. dirty balls...
an object ball shot at an angle generates this gearing motion without the aid of english. You're coming across it, the object balls inertia resists this spin you're trying to put on it, so it torques off its natual line. You either put opposite spin on the cue to compensate, or you adjust your angle to where it will be thrown to.
If you really want to make your straight shots... use draw. This way you will be throwing the ball forward, regardless of how much english you unintentionally place on the cue. You may still have a little left or right left on the cue ball, but you should have more draw.
New players who are used to shooting center ball all the time, are going to fall prey to this until they figure out how to shoot a good stop shot. My girlfriends solution to a stop shot (without knowing how to draw)? Shoot hard so the cue ball jumps to the object ball, and does not generate forward spin from the friction of the felt. She doesnt know she's doing it, but this is why she does it.
Like many previous posters have said... most peoples shot problems can be fixed by revisiting their fundamentals. (I would also add in some strength training)
my .02
Josh