Paul Hiebert examines it:
In a sense, when a teenager deems a person or idea "So random!", they are being dismissive of that person or idea. The teen who utters this word after being confronted with something unfamiliar—an event that doesn't resonate with his understanding of the universe—is in a way regaining control by restoring order. What is random is folly, and therefore not a threat. In other words, it's comforting to consider our beliefs and perspectives as logical—they make sense, after all—while any beliefs or perspectives outside of, or in opposition to our own, must therefore be chaotic, confusing, random.
The Dish 