ivyleager,
Did your chlorine level get towards zero over the winter or were you maintaining its level? If it went to zero, the soil bacteria could then grow in the pool consuming the CYA, and when you opened up you'd have a lot of ammonia in the pool (the bacteria break down the CYA into ammonia and carbon dioxide). So you might find a HUGE chlorine demand upon startup as you have to oxidize all the ammonia.
If you maintained FC levels over the winter, then the drop in CYA is more of a mystery.
Richard