It's not a phoby, exactly... More like an illness, you could say I am byoyomically challenged - or, simply put, playing in byoyomi equals loss of 5 to 8 ranks as far as I am concerned. Under time pressure, my poor skills just collapse; it's like a vicious circle: I can't concentrate because I know I have to read and think fast and I can't because all I do is tell myself to concentrate, so I usually just use my 30 seconds time with "crap time runs out time runs out must move fast must think now no time *slam*"
I've heard of players who perform their best when in byoyomi and find enjoyment in actually let the ing-clock claim "nine" before slamming down the stone and on the button. Probably just myths though.