Strange, I "like" your shot and as a result the average count drops. That's unfair!

you mean the score dropped? I guess it's because of the 500px counting system which has something like a decay time for scores... at fixed intervals, the score for a photo will drop. It's to make sure that there's always new awesome pictures in categories like 'popular' etc... you can look it up somewhere in the FAQ section if you're interested :)