changes in the milk due to the seasonal changes... and\or changes in the goat's diet. It can also have an affect on the taste of the milk too.
Goats, if they have a free range, can get into most anything... the one's we have love the pecan trees and rose bushes... when they get into those, things change.
If it is your goat... you may know this. If it is not, you can find out from the farmer you get the milk from.