The STEEM Blockchain has rules put in place to determine when to pay SBD and/or STEEM. And is based solely on the SBD Debt ratio. (To which you can check here: http://www.steemreports.com/steem-sbd-info/ and is currently at 6.75%

Currently the rules are:

• When the STEEM/SBD debt ratio is between 0% and 2%: The Blockchain will pay 0% STEEM, 100% SBD.
• When the STEEM/SBD debt ratio is between 2% and 5%: linearly changes from 0% STEEM, 100% SBD to 100% STEEM, 0% SBD.
• When the STEEM/SBD debt ratio is greater than 5%: 100% STEEM, 0% SBD.

As seen on the link above, the current SBD Debt Ratio is greater than 5% (currently at 6.75%), thus the rule is to pay all liquid rewards in 100% Steem 0%SBD or simply ALL in Steem. This rules however will be changed on the next HardFork and will be increased to 9-10% instead of the current 5%+.