During Animation only the presentation layer gets changes while the modal layer of the view stays to its original state. In your case you are not preserving the state of presentation layer when animation is finished and Modal layer appears which is in it original form. which seems like the Jerk effect.
Use the following line after "animationgroup.fillMode=kCAFillModeForwards;" to preserve the presentation layer state and the jerk will go away.
animationgroup.removedOnCompletion=NO;