Despite being a native Brit, I've always found it an oddity that words like "tutor", "tube", "tumour", and "duty" are pronounced as "tyutor", "tyube", "tyumour", and "duty" in British English. For me, this doesn't seem phonologically simpler/easier at all, and given that it seems to be absent in American English (possibly other forms of English too?) for these sorts of words (beginning with "tu" or "du"), I would suppose it's a somewhat recent innovation in historical linguistic terems, almost surely after the Middle English period. So that's my first question: when and where did this pronunciation originate, and as a bonus, why?
Now, there are certain words that have this "y" sound inserted even in American and other forms of English, though I can't think of any where it's inserted in the "tu" or "du" sound. e.g. "beautiful", "cute", "futile". Notably, this pronunciation seems much more natural to me in these cases, unlike in the "tu" and "du" cases. Of course, the pronunciation of these words in the original Latin (or even the intermediary French when applicable) suggests that this innovation happened within English, and most likely within England itself. (N.B. I can't think of any Anglo-Saxon words with this mode of pronunciation right now, but quite possibly there are some too.) So, my second question is: is there some clear linguistic/phonological reason why the insertion of the 'y' sound is more natural in the non-"tu"/"du" cases, and did this phenomenon originate earlier in the English language?