SBD are a debt instrument, since they can be exchanged for USD$1 value of Steem. There are limits to stop this debt becoming too large in relation to the overall supply of Steem (measured in dollars). 

The SBD print limits, expressed as a percentage of the overall supply of Steem, are currently:
* Between 2% and 5% the supply of SBD is gradually reduced and replaced by liquid Steem in 50 / 50 payouts. The change from 2% to 5% is a linear progression.
* At 5% there is no longer any SBD included in post payouts, you receive liquid Steem instead.

The recent fall in the price of Steem has reduced the overall Steem supply (measured in dollars) such that the 5% SBD limit has now been breached. No more SBD will be printed until the outstanding SBD returns to below 5%.

According to steemd.com the current market value of Steem is $258,583,040 and the amount of SBD is 15,683,596 (note the system assumes 1 SBD = $1). So it looks like (the 3.5 day average witness feed price of) Steem has to rise back to around $1.08 (with other things, like SBD volumes, remaining constant) before SBD rewards will restart.