Question: Is there a formal logic symbol for "why"? For example how would you formulate "Why is 2^4 > 4^2?" Could that be formulated in pure symbols of logic if possible?

Also, the phrase "what is" can it be described with logic symbols? Is it just "=" equals? So for example you walk down the street and hear a bang. And you conclude it's either a car crash or a kid popping a balloon. How would that be expressed logically with symbols?

Answer: 
No. Formal logics deal with concrete propositions, not speculative discussions. Speculations have huge levels of nuances and are usually performed with narrative language. In fact, papers or scientific documents use mostly narrative argumentation, except for the concrete formal propositions, which are written using formal logics.
To the extend that 'what is' is '=', in systems of logic that trace deduction literally, such as proof theory, implication (⇒) captures the basic notion of why. What lies to the right of the arrow is true because of something that lies to the left.

But Classical logic simplifies the meaning of implication into something unnatural by insisting all true statements are equal in meaning and all false statements prove anything, regardless of referential content.

People who want to talk about semantics then usually introduce a symbol to recapture the natural meaning of implication. So I would suggest that the semantic implication symbol 'turnstile' (⊨) really at least intends to capture the natural sense of 'why?'

When these are interrogative, what is implying or what is equated is a free variable. "Ǝ X: X ⊨ 2^4 > 4^2" means "There are collections of facts and deductions that entails the fourth power of two exceeds the square of four, we will call some one of those X." (In this case there is no X, because the right-hand-side is simply always false.) Then using X anywhere is, in effect, asking why.