I would be inclined to agree that there was no market manipulation if the volume was more sustained and lasted more than a couple of days. A spike of over 300% of average volume, however, represents an anomaly which might be coincidental, but warrants further investigation. Without any outside factors (such as a major news article on Steem/SBD, a new partnership with a third party, etc.), the volume increase seems unwarranted. Why the sudden new interest in SBD and why doesn't it last for more than a day or two? Without combing over the daily trading data to determine who is making the trades, it is impossible to definitively say whether the market is being manipulated by a set of individuals.

As a counterargument, I would expect new investors to invest directly in the more popular crytpocurrency, Steem, rather than directly into SBD. This belief stems from the fact that Steem tokens have greater utility (they can be converted into Steem Power) and visibility (they appear on the top 50 list of coins on coinmarketcap). Although SBD can be converted into Steem at a 1:1 ratio, the removal of the conversion option from the Steemit website has made it more difficult to do so. The reverse is not true. Steem cannot be traded for SBD directly. The effect of this is a one way cap on the price of Steem without a cap on the price of SBD (so Steem should theoretically never be worth more than 1 SBD for a prolonged period of time as people would take advantage of this arbitrage in the market to convert their SBD to Steem and immediately sell their newly acquired Steem back to SBD for a profit). The price limit does not, however, translate into an expected market cap for SBD (which will be a product of the demand for SBD offset by the constant supply via rewards for posts).

Although the supply of SBD is significantly lower than Steem, SBD has significantly less utility than Steem (it can't be converted into Steem Power, it can't be used for voting, etc.) and thus would be less inherently valuable. What creates the premium frequently observed is the guaranteed conversion to Steem coupled with the limited supply. I am inclined to argue that SBD does not have a value without it's being tied to Steem (as it can't be used for voting or to directly get Steem Power). With all that being said, you may very well be correct that the spikes in volume represent organic growth. I, however, would expect volume to increase more gradually if that were the case.