Why is electronegativity increasing with oxidation state increase?
Yes absolutely, electronegativities are hardly static values when you start combining elements and forming ions or molecules. 

Electronegativity is simply a measure of an ability of an atom to attract electrons in a covalent framework. 

So keeping in mind that like charges repel (electrons repel electrons) ... it follows that elements in a high oxidation state are more electronegative - i.e. more able to attract and stabilize electrons. High oxidation state elements have essentially been "robbed" of their electrons and likely bear some degree of partial positive charge ... this is "good" for attracting and stabilizing nearby electrons. 

Conversely consider a lone pair - which is defined to have an electronegativity of 0. Lone pairs cannot stabilize other electrons and don't attract other electrons to themselves ... the electron-electron repulsion would be too great.