Why do transition metals display variable oxidation series?

Transition metals have electrons of similar energy in both the 3d and 4s levels. This means that one particular element can form ions of roughly the same stability by losing different numbers of electrons. Thus, all transition metals from titanium to copper can exhibit two or more oxidation states in their compounds. 

Oxidation states of some Transition Metals: 
Titanium- +2, +3, +4 
Vanadium- +2, +3, +4, +5 
Chromium- +2, +3, +6 
Manganese- +2, +3, +4, +5, +6, +7 
Iron- +2, +3 
Cobalt- +2, +3 
Nickel- +2, +3, +4 
Copper- +1, +2 
When Transition Metals form positive ions they loose their electrons from the 4s sub-shell first, then the 3d sub-shell