They are no longer backed by $1 worth of steem all the time. When the total steem dollars equals more than 10% of the market cap of steem they are worth less than $1 of steem.

If you look at debt ratio on this site: https://steem-supply.john-at-me.net/

It is capped at 10%, so steemdollars are worth $1 maximum with essentially no minimum value now, just a value that ensures all SBD are worth less than 10% of the steem market cap.

Also you can't convert on steemit anymore, try https://steemworld.org/@paws1t1veev in the options if you really want to convert.