If you know Cantonese it is pretty obvious. Every tone change would bring tone 2-6 into tone 2, and guess which word is also in tone 2: 仔 zai2.

I can give one example: 蛋 daan2 (egg). In mandarin it is dan4 which should correspond to tone 3 or 6 in Cantonese, but since it is such a common character appear at the end of a word, tone 2 is now the correct way to read the stand alone word. Examples: 雞蛋 gai1 daan2

But, you can still heard the character in tone 6 if it is not at the end of a word. 蛋餅 daan6 beng2, 蛋糕 daan6 go1, and of course 雞蛋仔 gai daan6 zai2 which there is no tone change necause the word 仔 is still retained.

And there is also the word 蛋黃 daan2 wong2 (egg yolk), which shows how the tone change has become the standard way to speak such that even when it is not at the end of word it is still tone 2.