BK2K has what is called "Auto replay percentaging" or something like that. I would guess that first, your machine is set to award an extra ball for a replay score rather than a free game.
Once you hit the free play score (say, 1M), the machine automatically raises the score needed to achieve the free game on the next game., If it is reached again, it will go up again, and so on. It is a way for the machine to keep someone from being on the same game all day long because they can score a certain point value.
If you miss the point value set for a free game during any certain game, the machine will then lower the score needed on the subsequent game after that until it is reached. I think this started on High Speed, and honestly is a pretty cool feature.
Chris