In linguistics this is called a coarticulatory effect, and it’s caused by needing to move the articulators between two positions rapidly. As such, it can be thought of as a kind of “hardware” limitation of humans, as opposed to a “software” limitation of any single language. Whether other languages would have the same sounds in sequence is the main factor.

The “ch” affricate (which is t͡ʃ in the IPA) is a mix of a voiceless alveolar stop component and a post-alveolar fricative component. Because “y” is palatal, you end up getting that post-alveolar fricative component through coarticulation.

Edit: here’s an explanation without the jargon:

• “t” in English is produced by your tongue contacting the ridge behind your top teeth.

• “y” in this context is produced with the tongue sitting near the palate (significantly behind the ridge used for “t”).

• The English “ch” sound is actually a mix of two sounds: “t” and “sh”, in rapid succession.

• The “sh” sound is produced between the places where “t” and “y” are produced.

So, if you have a “t” and a “y” in quick succession, your tongue has to move quickly between a couple different spots – and crucially, through the spot which produces “sh”:

“t” -> “sh” -> “y”

And because “t” + “sh” equals “ch”, you get the “ch” when producing this sequence. You can of course articulate things carefully and not produce it – but in common, quick speech, that’s why it shows up. Singing isn’t different from speech in this regard.

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In British English, we do this a lot with ‘u’ sounds. For example, in London we take the Tube, which Americans pronounce ‘toob’ and we pronounce ‘choob’. Strangely enough, almost all English speakers do this with the word ‘train’ and other ‘tr’ words without even knowing it: the first sound is not ‘t’, it’s ‘ch’.

Both British and American English speakers do something similar with ‘s’. For example, ‘issue’ is pronounced ‘ishoo’ except by people with very strong RP accents (that is, posh people), who say ‘iss-yoo’.

It’s a similar phenomenon to the ‘c’ before ‘i’ and ‘e’ transforming into ‘s’, e.g., in ‘science’, ‘ceiling’ and probably hundreds of other words (‘cœliac’ is a particular favourite). Italians do something similar, but they make it ‘ch’ as in ‘ciao’.

Brazilian Portuguese speakers change ‘t’ and ‘d’ to ‘ch’ and ‘j’ respectively before ‘i’ and ‘e’ sounds. For example, the word ‘de’ meaning ‘of/from’ is pronounced more like ‘juh’. In Portugal, though, they use a hard ‘d’.

So, in summary, ‘e/i’ and ‘u’ sounds all have a tendency to transform the preceding consonant, especially if that consonant is ‘t’, ‘s’, ‘c/k’ or ‘d’, more rarely with ‘j’ and ‘z’ and sometimes with ‘b’. If and how they change varies by language and dialect.

(Note: I’d have done this with IPA but I’m not sure about Lemmy’s support for it and I didn’t want to type it all out and have it be a load of non-loading characters, hence my approximations of the pronunciations. Apologies to any linguists out there!)

Something similar in German is if a verb ends with “st” followed by “du” (you), it’s squashed down to “-stu” …

from a song “Sag mal, weinst du oder ist das der Regen…” becomes “weinstu”

“hast du” …“hastu”

In French we have liaisons, where in basically every word ending with an s followed by a word starting with a vowel, the s is pronounced “z”. For instance, mes amis (my friends) is pronounced “mez amis” instead of “meh amis”, des oiseaux (some birds) is “dez oiseaux” and not “deh oiseaux”. One of the many things that makes French hard to learn for foreigners.