Wideangle lens - strange behaviour

I've got a Nikon 14-24mm f/2.8 lens. I did some long exposure architectural shots recently using a tripod, wireless remote and mirror lockup - some were at 16mm and some at 14mm.

All of the 16mm shots were sharp both in the centre and at the edges . The 14mm shots were sharp in the centre but they were all extremely soft and/or distorted at the left and right edges (not the top and bottom). Has anyone had a similar experience with this lens?