Assimilation is a common phonological process by which a speech sound is changed due to the influence of its neighbors. This happens a lot in the flow of speech especially when one speaks rapidly. For example, best man is typically voiced as [bespmæn] as [t] assimilates to [p] due to the influence of the following [m]. Sometimes the sound modification becomes permanent, i.e. in the word import, n changed to m due to the the adjacent p.

However, assimilation rarely occurs in Mandarin and Cantonese. The reason is probably because of the monosyllable nature of the Chinese characters. Since both Mandarin and Cantonese are formal languages, there are armies of school teachers who are ever ready to 'correct' the students' 'mispronunciation' if assimilation ever take place. Taishanese, being a rural colloquial language, is less regulated thus becomes rich in assimilation.

Anther reason may be due to the fact that Taishanese has a large number of so-called zero-initial syllables (usually corresponds to syllables in Mandarin and Cantonese with the [d] initial), i.e. 凳 stool/chair (Mandarin deng4; Cantonese /dang3/) is [aŋ21] in Taishanese. A zero initial may acquire an initial if they follow a syllable with an ending consonant. For example, 担凳 to carry one's chair [am33 aŋ21] is usually pronounced [am33 maŋ21] in the flow of speech.

Syllables with [h] initials also have the same tendency. Here's a case of a non-optional permanent assimilation: 咸虾 salted shrimp paste is not [ham33 ha21], i.e. what it supposed to be, but [ham33 ma21] -- the [h] initial of the second syllable was dropped and replaced with the ending consonant of the first syllable [m]. This is a case of silent h, i.e. h is always silent in Spanish, and in some English words as well (hour, honest, herbs, etc). I tripped up on this word. Salted shrimp paste is a very common Taishanese condiment, and I had not the faintest idea that it was made of shrimp for a very long time because of its [ma] assimilation (which makes it a salted horse).

Here are several non-optional assimilation  (the sound corruption has stabilized and become the standard) examples: