How the substitution of hydrophillic glutamic acid with hydrophobic valine cause sickle-cell anaemia?

in low oxygen conditions there is an increase in deoxy haemoglobin, which exposes a hydrophobic region on a patch between two helices of the molecule, and in the case of HbS (the haemoglobin with the valine) this causes polymerisation and the sickle form of the red blood cell, which get trapped in small capillaries causing crises