I'm wondering what mechanism puts a y sound (IPA /j/) into words like coupon, which presumably had none when it came into the language. French pronunciation would seem to indicate it would be pronounced [ˈkuːpɔ:n], not [ˈkjuːpɔ:n], but many people pronounce it the latter way.
Similarly, why do we have such a difference in a word like duke, which can be pronounced [djuːk] or [duːk], when dude only ever seems to be pronounced [duːd]?
addendum
An even more puzzling example, brought up in the comments below, is that of Houston, which is pronounced with the /j/ when it refers to the city in Texas, and without it when referring to the street in Manhattan.
Best Answer
Just guessing, but perhaps the /ju/, which came by breaking a high front rounded [y] in words borrowed from French, came to be regarded as a high prestige form. Then, hearing /u/, some English speakers interpreted that as a mistaken or low-class way of saying /ju/ and decided to correct it in their own pronunciations. If that's right, the /ju/ from earlier /u/ should turn up in words most easily interpreted as having been borrowed from French. Folk loan phonology.