What causes a hemmorage in the eye following a retnial detatchment surgery.?

Vascular leaks: The retina is very thin, very fragile but richly endowed with blood vessels and it sits on another carpet of blood vessels. Re-attachment surgery has to restructure a detached retina in the presence of all of this vascular richness and fragility and sometimes smaller vessels can leak or even vessels at the site of entry into the eye. This will clear usually with time.

Many causes: It could be residual blood from the retinal tear. The eye is very vascularized so it can be from many sources. Usually it will go away if left alone, but you should defer to what your surgeon says.