Sasha is derived from Alexander via its dimunitive, but obsolete form - Aleksashka - shortned to Sashka, further simplified to Sasha as per established name for

Sasha is derived from Alexander via its dimunitive, but obsolete form - Aleksashka - shortned to Sashka, further simplified to Sasha as per established name format of Masha (Maria), Dasha (Daria), Pasha (Pavel, Paul), Glasha (Glafira), Natasha (Natalia), etc.