The picture tells everything. Since August the 6th, there was a big decrease of active users on Steemit.
Theory 1
My first theory is that the people see the falling STEEM price and think that they can't make any money anymore with blogging - here on Steemit. As the people see the sinking ship, they just leave it until it's magically repairing itself somehow.
So the STEEM/BTC price was falling down since the ~20th June. My explanation for this is, that people move their assets into BTC, as it's the strongest cryptocurrency right now.
But somehow the people left Steemit within the days of August the 6th and August the 8th. The reason for this, could be the typical panic moment in any crypto, when a big price drop happens. Just when a situation like this occurs, it should normally be a healthy sign to buy. But seeing a drop of so many users just because of the price crash? Makes me nervous...
Theory 2
The unactive accounts were bots to generate STEEM via auto-upvoting or similar. Those accounts got detected by a 
Theory 3
Resulting from the unlikelyhood of Theory 2, a new theory is that SteemData is just wrong. Why would so many people leave within a few days just because of the STEEM price movements, which is the only logical explanation for this.