According to my dictionary, this is the origin of Pronounce:
ORIGIN late Middle English : from Old French pronuncier, from Latin pronuntiare, from pro- ‘out, forth’ + nuntiare ‘announce’ (from nuntius ‘messenger’ ).
So it seems it may be more a case of pronounce gains an O, rather than pronunciation losing it.
Having said that, as someone who learned English as a foreign language, the reasons and origins of the grammar, spelling and pronunciation is still much of a mystery to me…(Worcestershire is my favorite example)