The very tab where you read the current hydration requirement tells you why. Expand it and look at the text, not just the mouseover number.
Hydration requirement is a weighted average depending on three factors: temperature of the biome you build in, humidity of the biome you built in, and time elapsed since it last rained in the world. If you have many minecraft days passing without rain (for example, if you build in a desert, or have rain switched off via NEI cheat mode), then your farm will be very dry and require huge amounts of water to keep going. As soon as it rains, that value will start to go down and the longer it rains, the further it will fall. It will go down all the way to like 50% if it rains enough.
Also, updating Forestry might help. In newer versions, the maximum hydration penalty from a lack of rain was capped around 2500%, instead of climbing almost infinitely.