[Nonfractal]

The armature has momentum.
(Angular momentum)
When you take an inline motor from full throttle to zero, that momentum is transferred to the chassis, putting torque on the chassis which lift one rear wheel pushes the other down to the track. 

You can see the effect clearly if the motor is revved and braked sharply outside the car.

The same can be seen on a BMW boxer engined motor bike. 
The crankshaft is inline and transfers the momentum to the frame when the throttle is blipped. 

I have to say though, that the effect in a slot car is rarely enough to upset the handling in any severe kind of way.
I run king30 motors inline in AllSlotCar F1 cars and have never suffered with "jumping sideways" as you describe. 

Alan