As you point out, during their maturation human red blood cells (erythrocytes) eject their nuclei, together with the rest of their organelles.  

The exact mechanism behind this process remains incompletely understood, but it appears to be specialized version of non-symmetric cytokinesis (cell division without mitosis).

We does appear to be the case is that that process normally involves the association of the maturing erythroblasts with the surface of macrophages in what are called erythroblastic islands.  

Signals between the macrophage and the developing erythroblasts, dependent upon the action of the Rb (retinoblastoma) gene in the macrophage, appear to be critical (webref).  

The interaction between macrophages and erythroblasts involves the Emp protein, which is located in the macrophage's plasma membrane (webref).   

Further details remain to be determined.