Hello.

It’s directly related with the steem median price.
Your SBD reward is printed from reward pool (in STEEM). STEEM are converted in SBD with STEEM median price
When STEEM median price goes up, reward goes up (because SBD is more printed)

When STEEM median price goes down, reward goes down (because SBD is less printed).

I’ve created a tool to follow financial metrics.

http://steem-supply-steem-supply.193b.starter-ca-central-1.openshiftapps.com/