It is a common process in development of language over time. When two consonant sounds are adjacent, the first is transformed to other sound. Native speakers can pronounce a word easily, but same word comes to non-natives, they become confused and pronounce it wrong some time.
The letter “d” makes the /j/ sound usually when it is followed by letter “u”. It happens generally when letter “u” connects the root or base word ending with a “d” to a suffix. Such words are commonly derived from Latin origin. Below are the few examples:
- Schedule
- Graduate
- Residual
- Individual
- Gradual
There are many other words that have /j/ sound like words education. Try to say ‘Ed-you-cay-shun’. It is very difficult to pronounce d and u distinctly in a word. That is why that these two consonants d and y just slip together and produce /j/ sound.
When we pronounce word duke or due, we actually have two vowel sounds after letter d. We pronounce it like di-ook, or di-oo so the sound that is following d is /i/ not /y/, because /y/ sound is more similar to sound /j/. Therefore, it depends on sound that is the following letter ‘d’.