The reason that second one's so relatively common is a combination of a normal mistake with a spellchecker typo.
"Defiantly" is the obvious spellchecker suggestion for "definatly", which is a close enough match to the pronunciation of "definitely" - especially in accents where the penultimate syllable, "net", comes out closer to "nut" - that some people are going to come up with it in an honest attempt to spell the word.
"Defiantly" is the obvious spellchecker suggestion for "definatly", which is a close enough match to the pronunciation of "definitely" - especially in accents where the penultimate syllable, "net", comes out closer to "nut" - that some people are going to come up with it in an honest attempt to spell the word.