This is found principally with soft metals.  Two soft ligands in mutual trans position will have a destabilizing effect on each other.  The effect is also found with borderline metals in the presence of high trans effect ligands.  For example the selenocyanate ion trans to the soft carbon dioxide in trans-Rh(PPh)(CO)(NCSe) bonds via the nitrogen, the harder of its two donors. The phenomenon may be explained in terms of a trans influence: