The story
An alteration of -our, arising in the 15th and 16th centuries through analogy with words such as saviour (Middle English saveour, savour) and paviour (Middle English paver, pavier) which varied between forms with an intervening -e- or -i- and those without due to differing adaptions of their Old French sources (sauveor and paveor, *pavier respectively).
Etymology from Wiktionary, CC BY-SA 4.0.