Sometimes different things can bring out anxieties in us, even anxiolytics if we are in the right [or wrong] frame of mind. [Though it is much more difficult to go into anxiety or stay in an anxiety state for long with excess GABA running through your brain.] I agree with Krunky, it is an especially anxiety-producing time in the world. Aside from this, everyone is different, you may already have an anxious personality, and although the Kava superbly calmed it, there may have been a slight rebound effect mixed with your already anxious and over-thinking [and now sober] personality the next day that spurred an anxiety attack.
As example, I do not necessarily think Kava has a physical withdrawal that for sure will give you rebound anxiety, as there have been times after frequent heavy kava use I have woke up the next day, went to work, been busy, got home, passed out, and felt no real difference. There have been other times that just with more on my mind or less to do, I become psychologically aware that I was using heavy Kava which calmed or distracted me from anxieties, and now I am dry, which can lead to an almost self-induced psychological rebound anxiety withdrawal state - that usually clears up as I go through my day and realize I will be just fine!