Airliner accident investigations are almost always a slow process and this one is no exception. According to this CNN article, the plane had rudder problems during a preflight check. A mechanic did a reset on the yaw dampener computer and the problem seemed to be fixed. Maybe not.

A yaw dampener is a form of artificial stability system that many aircraft need to make them more controllable in that axis. Some aircraft tend to have weak yaw stability, so a yaw dampener reduces the pilot workload and makes for a smoother flight. If the yaw dampener failed on Flight 587 as the plane encountered wake turbulence, it might've caused the pilots to overcontrol and perhaps overstress the vertical stabilizer and rudder.

I've heard some speculation that there was flutter on the plane's rudder. Flutter is a condition where an out of balance control surface starts oscillating rapidly, causing a lot of stress on the attach points and other structures. It's a very dangerous situation. There was an F-117A lost at an airshow a few years ago when it developed aileron flutter that ripped the wing off. Flutter seems unlikely to me unless someone did something to unbalance the control surface during maintenance. Off hand, I don't know of a single commercial airliner accident caused by control surface flutter. But then, losing a vertical stabilizer is an extremely rare event as well. This may be a first.