I think it's part of a larger pattern in English in which long words truncated at the second syllable often pick up an -o ending instead of retaining the natural vowel sound and spelling that occurs in that second syllable. Here are some examples:
aggro [aggravation/aggression]
ambo [ambidextrous]
ammo [ammunition]
camo [camouflage]
combo [combination]
convo [conversation]
distro [distribution]
invo [invitation]
limo [limousine]
mono [monaural/monophonic]
provo [provisional]
Valpo [Valparaiso University]
When a phenomenon occurs repeatedly, as this one does, it suggests that the tendency to favor -o endings in truncated words over -a, -au, -e, -i, -ou, and -u endings reflects a genuine, though informal preference in the language, at least for the moment. I don't know whether this particular phenomenon has been discussed in a scholarly setting, but I wouldn't be at all surprised if it has.
