I believe this was due to C of G issues with modern engines. Original 737s had slim engine nacelles and short undercarriage (-200). Engines have grown wider and more efficient but the undercarriage is the same length. In order to fit the new engines and still be able to sit on it’s wheels they are hung forward and higher, thus moving CoG forward and introducing a level of instability. Saw this on a documentary about it I think, forget which one. Makes sense though..