The Klout score is not based on your activity level but on how your audience reacts to your content. You may not have changed your behavior but if your audience's response has changed, then that will be reflected in the Score.

The Klout Score is in essence “PeopleRank” and we take data from social networks in order to score users on the basis of:
-How many people you influence
-How much you influence those people
-How influential those people are

The end result is your Klout Score – a measure of your influence on a scale of 1 to 100. We take the past 90 days of data to generate your Klout score. As that 90-day window moves forward, some of your days with high engagement may not reflect right away on your profile. Your Klout score will recover as we analyze new data.  We are currently working on a project that will give you specific information whenever your Score goes up or down so you know exactly what caused the change.