THe only reason SBD didn't stay at 15$ for a long time is the same why it got there. BTC
First, BTC soaring up brought SBD up to 15$ as Koreans pumped massive amounts of money into SBD. upwards of 10m in 24h volumes.
When BTC started it's downtrend so did SBD, and all other altcoins followed along as you might already know.
I see no reason why the next BTC bullrun wouldn't have the same effect on SBD that it had a year ago.