The corruption/evolution may be one and the same. There is a theory that there are two probable reasons, both working together simultaneously: one being a rebellion against "them" (anyone perceived to be a class above, including teachers in particular) by deliberately speaking unconventionally, and this is still going on (sickth in line, how are you - I'm good, lore and order, uzz not us, etc., etc. being very recent developments). The other being a tendency for obscurity as in offside and nearside of a two sided thing (not four sided - there is no onside or farside) that is in fact easily described by referring to left and right - but then everybody would instantly understand what was being talked about. But there are distinct limits to how far from the norm you can go without literally no longer speaking what will be understood by those who speak the language. Speech impediments also appear to have an effect (origin of the glottal stop ?), as do the way new rulers speak. Apparently, Dutch influence (William) has caused the letter i in English now to often be pronounced as in the pronoun I, rather than as in sing - the first being common in Dutch but the second common more or less everywhere else. The English alphabetic name for the letter i is similarly unusual while the alphabetic name for the letter e in Dutch is identical to the way native English speakers tend to mispronounce the letter e in foreign words - as ay (as in day) instead of e as in bed (most common in other languages, although there are exceptions, but more or less never as ay).