NATLANG: Long/Short Variations in English

Hey everyone,

I was wondering if anyone had an explanation for some long/short variations
of words in English.  Here are the words:

"a": /@/ ~ /a/ vs. /e:/ (< /a:/)
"the": /D@/ vs. /Di:/ (< /De:/)
"to": /t_h@/ vs. /t_hu:/ (< /t_ho:/)
"you": /j@/ ~ /ja/ vs. /ju:/

What process(es) caused these originally allophonic pairs to exist?

- Rob

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