What can make an aircraft swing?
The rudder itself, Thrust assymetry (short pop or otherwise) Dragging Brake, Yaw Damper input, Atmospheric condition (wake or general) Other flight control assymetry.
As did 'L337', so did I experience a sharp swing on T/O about a year and a half back. It happened in ZRH in a 737-300.
Before the investigation could be completed, the incident had reached these pages, accompanied by lots of polemic "Problem rears its ugly head again"- etc. As this is a very sensitive subject, I didn't want to comment until sure of my facts. maybe now is the right time.
In my case, the rudder pedals were not displaced to the left (the direction in which the aircraft swung). The swing was sharp and of short duration. We were close to Vr and engine instruments were all OK (my first thought was a surge)- the ship was quickly under control and we took off.
I have to say that Boeing were super - organised an immediate telephone conference and asked many pertinent questions, as well as filling us in in great detail on the history of the problem, as well as possible failure modes. We pulled the flight recorder and had a read out made as soon as possible.
It turned out that the swing was visible on the recorder - the ship swung a couple of degrees (it felt like a lot more because it happened fast) and there was even a half degree of left bank shown as the left oleo compressed. The rudder position was straight ahead (as was the pedal position) and immediately the Yaw Damper corrected the swing with a kick to the right - ie. working as it should. We went through a list of other possible reasons and all we could come up with was a local atmospheric effect - possibly caused by a slight quartering tailwind being deflected by a small forest.
In my case, the ship was not to blame. Before any result could be obtained however, it had received bad press. The tendency is to put incidents into neat pre-conceived categories and apportion blame with too little knowledge sometimes. I hope this helps to put the record straight, as far as the ZRH incident was concerned.