I snagged the Dec/Jan 2011 issue of Professional Mariner out of the officer's lounge the other day. There is an interesting article by St. Lawrence River pilot Capt. Hugues Cauvier; "Effects of the ship-generated sideways current".
A ship moving sideways generates its own current. If the ship then moves ahead or astern, part of the ship remains in the current while part moves out of the current.
Jerry, a tug captain I sailed with sometime ago used to describe the currents and back eddies on the Inside Passage as "fast water" and "slow water", the context being having the tug in slow water and the tow in fast water was a bad thing.
A ship moving sideways generates its own "fast water" Moving then either ahead or astern is going to poke the bow or the stern into "slow water" This would cause the ship's heading to change. I have observed heading changes at slow speed which were not what I expected. This explanation seems reasonable. Next time I pull the ship out of its' own fast water I'll be watching.