My personal theory about this mispronunciation is that urdu/hindi speaking folks tend to prefer elongating vowels in the beginning of the urdu words whereas they tend to shorten the vowels at the end of urdu words. But we Punjabi speaking people are totally opposite to it, we tend to shorten or sometimes even totally discard the vowels at the beginning of the words whereas we tend to elongate the vowels at the end of the words.
I give you example, urdu word for work is "kaam", this "aa" shows the preference of an elongated vowel at the beginning of the word in case of urdu, whereas the same word in punjab is "kam" or can even be simplified as "km" because we punjabis of central punjab prefer to shorten our vowels at the beginning of the words. Similarly take the english word "punishment", the urdu speaking people will pronounce it as "saza" and it feels like the vowel "a" at the end is very shortened by them, whereas our punjabi instinct is to elongate the final vowel, we you will often notice that we will pronounce it as "sazaa" with an elongated "aa" vowel at the end totally opposite to urdu speaking people's pronunciation.
This is what helps us punjabi to pronounce foreign language words which tend to have either no vowel or very short vowel at the beginning of the words. For example the word "smart" has an "sm" consonant cluster with s and m constants put together without any kind of vowel, now this is the case what we punjabi speaking people love to have either very short vowels and just a cluster of consonants at the beginning of the words, so we have no problem pronouncing english words starting with consonant clusters like "school", "smart" with sch and sm consonant clusters at the beginning of the words but the urdu/hindi speaking folks have trouble with words starting with consonant clusters so they add their own "i" vowel to ease the pronunciation for themselves, hence mispronouncing these words as "ischool" and "ismart" etc.
It is just a different way of pronunciation preferences of vowels and consonants that developed in divergent ways in central india in case of urdu/hindi and Punjab in case of punjabi language.