I hope you're prepared for a very circuitous explanation of the meaning of this phrase....
First of all, a "grill" in this case is a decorative (?)* bridge of metal and gemstones worn on the upper front teeth. Here are a few examples. In AmE, if someone confronts you angrily, we often say that they are "getting in our face." Because the grills are worn on the teeth, if someone is getting in them, it means they are confronting you very closely - arguing with you nose-to-nose, more or less. "You all" is a slang phrase that means "a group of people." So "why you all in my grill" basically means "why are you people so confrontational and constantly arguing with me?" or "why are you so angry with me?" (And yes, in standard English there is a verb missing from "why you all in my grill," but slang doesn't follow the rules of proper grammar.)
*I add the question mark to decorative because most folks consider grills to be gaudy and unattractive. They are a fashion statement mainly among hip-hop types, wannabe gangsters, and thugs.
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