I have the 6600 and have had no problems with it kicking the fabric to the side .... and I've done a lot of sewing over mutliple seams/layers of fabric and no problem.
Don't second guess yourself ... in saying you are the problem!
I think more likely there is something wrong with the machine and/or foot.
I'll agree with all the others ... take the machine into the dealer and show them what's happening. Have them solve the problem for you so you can get on with loving your 6600!
Do you know someone else that has a 6600 that you could see how there;s works in comparison.