You start getting into the later half of the 4th quarter of the hard water season on most bodies of water and not only crappie but BIGGER crappie run much higher in the column. As a previous poster mentioned - food is one potential reason. 2nd is related to oxygen content. There are only two ways to introduce oxygen into water. 1) Aeration 2) Photosynthesis. Spring run off aerates the water days and even weeks before plants start doing their part. You may not have had water running down the holes last weekend but those fish are far from stupid.
Cuz that's what the Bos says...