This is a question I've had since I first played this game, maybe twenty years ago. I would say that it is random, given the nature of the game. However in my experience; the O-blocks never come alone (after you get one, the chance of getting another seems more likely). I also feel that you get the I tetromino just after you made a mistake.
I've been googling this for quite some time and haven't found a conclusive answer. Can somebody please help me? I've only played the game on the original Gameboy.