Steem dollars are set to peg back to 1 for 1 with the US Dollar. Any up-tick in the value of SBD should be temporary. I would personally suggest against buying SBD as an investment, especially right now since it is almost guaranteed to exchange back down to 1/1 with USD, meaning buyers should expect a 34% loss if they buy SBD right now.

Holders of SBD may want to sell now because you can just buy SBD back later for $1 instead of $1.34. What is causing it is likely that Tether USD is tanking, and people realize SBD can accomplish a dollar transfer over the blockchain just as well, causing a temporary price spike.