Steemit determines whether payout would be in Steem or SBD. Previously it was in steem alone but it was changed before the launch of hf20. I think they have a template for deciding this. Though here's my outsider knowledge on why this is done.

I think those running the platform monitor the amount of steem and sbd in circulation. When more steem and less sbd are in circulation, they would change the pay out to the currency with low circulation and vice versa. This is done to create a balance between the total amount of steem and sbd in circulation.

Demand also makes a currency's price to increase. When less of the currency is in circulation and theirs a high demand for that currency, such demand would push the price of such currency higher. Maybe this is what they're trying to achieve by changing pay out to sbd