Forced knockdown is one way knockdown may happen; some moves have a hitbox that, if successful, will guarantee that the opponent will go down to the ground without being allowed to perform ukemi or whatnot. This is particularly true of certain throws, overhead moves, or classes of moves (e.g., in Street Fighter III, you cannot perform a “Quick Rising” – its ukemi equivalent – if you are knocked down by a Super Art).
In many cases, this creates an ideal situation for using meaties; alternatively, an on-the-ground attack will basically be guaranteed. The defender is essentially helpless unless the aggressor does not do anything in time.