Q: What causes my reputation score to go down?

A: When some one with higher reputation scores Down-vote your posts or comments, then your reputation score will go down.

if someone with lower reputation scores than you Down-vote you, this will have no effect at all to your reputation score

source: https://busy.org/steemit/@breaktheinternet/steemit-common-questions-for-new-users-what-is-reputation-score-and-how-to-increase-your-reputation-score