I think this is related to the Cu damascene process employed for fabrication. The alloys of Cu which is used for interconnects are prone to this defect. The current process has a problem that when the current density is high, the kinetic energy of the electrons can, under certain conditions, can impart enough energy to the Cu-alloy molecules so that they clung on. This can cause certain bad effects such as an open-circuit or shorting with a nearby interconnect.. This is one of the reliability issues concerned with the Cu damascene process...
In order to ensure that this effect is nullified to a great extent, it is advisable to use wide metals...
I am a design engineer... So layout and fab guys, kindly correct me if i am wrong...