Two likely possibilities:
1. You reduced your utilization to 0 across all your credit cards. You need some balance to report on at least 1 CC. If a zero balance reports on all your cards, you will take a FICO score hit. 1-9% utilization is where you want to be on at least one card. All the others can be 0-9% with a marginal boost if all others are zero.
2. Some other factor reduced your score. If you reduced your utilization and if the myfico monitoring service shows your score reduction, it is because of something else. Myfico only provides score updates when a triggering event occurs. Not all score changes are reported. When a triggering event occurs (e.g. CC balance reduction) it recalculates your score taking into consideration ALL changes (including non-triggering event changes). Thus, a report showing a utilization reduction and a score change is almost always caused by this. If you look at the product feedback forum and the FAQ thread there you can look at the triggering events for the monitoring service and for each bureau.
Finally, one not so likely possibility:
1. If it is a very minor score change it is sometimes possible for lowered utilization - especially if your utilization is under 10% - to reduce your credit score by a couple of points. The all cards under 10% rule (or the more optimized all cards at zero and 1 card under 10%) works, but the sweet spot for utilization varies based on your credit profile. There is no one sweet spot for everyone. You might be able to get or lose a point or two by increasing utilization from 2 to 3 percent. I have never seen this happen but there have been earlier discussions about this. However, for practical purposes this does not really matter as it might buy you 2 or 3 points at best.
