It appears that this type of event happens on occasion with Boeing aircraft. You can read about it here.
http://www.bst-tsb.gc.ca/eng/rapport...3/a10q0213.pdf
"A nose-gear steering rate jam occurs when the feedback to the steering system is interrupted, causing the nose gear to continue turning (slewing) at a fixed rate until full travel is reached or until the rate jam is eliminated.
One explanation for a rate jam is trapped debris within the metering valve (slide/sleeve), which prevents it from completely closing. This prevention causes the nosewheel to change angle at a given rate, thereby causing an uncommanded steering input. A metering-valve rate jam is difficult to confirm, since it may not necessarily cause damage to the valve assembly, and the debris causing the jam may be flushed away through the hydraulic system fluid once the jam is cleared, leaving no evidence of a jam."