It was the procedure.
"The findings of the investigation by the National Transportation Safety Board (NTSB) were released on December 21, 1979.[1]. It revealed the probable cause to be attributable to damage to the left wing engine pylon that occurred during an earlier engine change at American Airlines's aircraft overhaul facility in Tulsa, Oklahoma. Evidence came from the flange, a critical part of the pylon assembly. It was revealed to be damaged before the crash, and investigators looked at the plane's maintenance history and found it was serviced eight weeks before the crash. The pylon was damaged due to an ill thought-out engine removal procedure. The original procedure called for removal of the engine prior to the removal of the engine pylon. To save time and costs, American Airlines, without the approval of McDonnell Douglas, had begun to use a faster procedure. They instructed their mechanics to remove the engine with the pylon all together as one unit. A large forklift was used to support the engine while it was being detached from the wing. This procedure was extremely difficult to execute successfully, due to difficulties with holding the engine assembly straight while it was being removed.
The procedure used for maintenance did not proceed smoothly. If the forklift was in the wrong position the engine would rock like a see-saw and jam against the pylon attachment points. The forklift operator was guided by hand and voice signals; the position had to be spot-on or could cause damage, but management was unaware of this. The modification to the aircraft involved in flight 191 didn't go smoothly; engineers started to disconnect the engine and pylon but changed shift halfway through; when work continued, the pylon was jammed on the wing and the forklift had to be re-positioned. This was important evidence because, in order to disconnect the pylon from the wing, a bolt had to be removed so that the flange could strike the clevis. The procedure used caused an indentation that damaged the clevis pin assembly and created an indentation in the housing of the self-aligning bearing, which in turn weakened the structure sufficiently to cause a small stress fracture. The fracture went unnoticed for several flights, getting worse with each flight that the plane had taken. During flight 191's takeoff, enough force was generated to finally cause the pylon to fail. At the point of rotation, the engine detached and was flipped over the top of the wing."
