Why always it is seen Maa Kali's tongue out side from mouth ?
My preferences goes to the following two explanations.
First one: where the act of sticking out the tongue is meditated upon as exterminating (drinking) the desires of existence:
Every time a Seed of Desire touches the ground, a new Seed of Desire is born in that very same place. Ultimately the entire earth has been filled with Seeds of Desire.
Seeing this and understanding fully well the tremendous import and significance of the all-pervasiveness of desire, the Gods became extremely dejected. In great alarm we all called to the Divine Mother for help. 'Oh Compassionate Kali, stick out your tongue and drink up all the desires of existence. Only your mouth has sufficient capacity to consume all desire! And when you will have digested all desire, then the Gods will be free from desire.'
This is why She shows Her very lovely, red, protruding tongue -- in order to make all existence free from desire.
Second interpretation: which targets Mahākālī as the first of the ten wisdom goddesses, the tongue is explained as evoking disgust on purpose.
Kālī’s gaping mouth and lolling tongue, her appearance and habits generally, are unquestionably repulsive to our ordinary sensibilities.
In Tantra, this is probably precisely the point. What we experience as disgusting, polluted, forbidden, and gruesome is grounded in and conditioned by limited human (or cultural) consciousness.
Finally, a third source, wikipedia, gives two different interpretations:
1) One interpretation of Kali's tongue is that the red tongue symbolizes the rajasic nature being conquered by the white (symbolizing sattvic) nature of the teeth.
2) The most widespread interpretation of Kali's extended tongue involve her embarrassment over the sudden realization that she has stepped on her husband's chest. Kali's sudden "modesty and shame" over that act is the prevalent interpretation among Oriya Hindus.
But personally, I prefer to stick with the first two sources.