Hmmm...
Well, it has to do with what is called torque of the engine.
Remember these are fighter planes with huge engines so any changes to the rpm of the engine will cause the whole airplane "roll" along the axis.
For those with cars (I do not know your age guys so I make some assumptions), those with big engines (V6, V
situated along the length of the car, if you step on the gas suddently, you will feel the car body rolling.
For those without cars, it is the wheel of the bicycle, when the wheel is spinning, it is very difficult to turn it to a diffenent direction, there is a powerful force causing it to roll.
It is the reason why the bicycle stays up when it is moving while we can not balance the bicycle while it is standing.
for those who have neither cars, nor bicycles, maybe you have a gyroscope, you can see the effect there, as long as it is spinning and your are trying to chance its direction, you feel the forces.
To cut the long story short, it is normal that the aiplane veers to the right or to the left (depending the way propeller turns) when you are accelerating (or decelerating).
You can compesate this by applying sufficient rudder to the opposite direction.
I hope it helps.
Happy take offs!
~S~