I'm not 100% sure, but I think the Leaderboard is a count of reactions(likes, etc.) to your posts. The only reason I could see your count going down is if someone unliked something they had previously liked. I'm not sure what the limit on likes are set to or how unliking a post affects your limit, but maybe someone unliked some of your posts trying to recover likes to use on other posts. I'm just speculating on something I know very little about, so take it with a grain of salt.