See here -
http://www.ancestry.com/learn/facts/Fact.aspx?fid=10&ln=RoxasX in Old Castilian became J at some stage. The surname Jimenez is still found as Ximenez on occasion.
As this name comes from a place name of Galicia, where a Portuguese dialect is spoken, and Potuguese retains the X (pronounced SH), this may explain the old spelling.
Rojas/Roxas could refer to reddish-coloured earth, I suppose.