Scrambling is term used in the world of wrestling and can be broadly defined as the act of rapid transitioning during unorthodox situations (e.g., an incomplete takedown) that is created by applying improvised skills combined with good technique. Old school wrestling coaches have not accepted it, since it does not involve applying well-defined techniques, but instead relies on the wrestler's instinctive, unrehearsed technique (e.g., movement, speed, balance, positioning).* Without a doubt, the concept of scrambling is very applicable to BJJ and submission grappling (although BJJ by it's very nature is already improvisational).
However you specifically defined scrambling, you know what it is when you see it ... and it's a gnarly badass sight to behold. Check out the latest scrambling clip from the wrestling madmen at flowwrestling:
* We made that definition up based on our research and knowledge, but we're by no means wrestling experts here (just BJJ experts). Still, we're pretty sure we're dead on.