I think its safe to assume that when theres a net outflow to the exchanges that those users are essentially leaving the STEEM market. At least to the degree of the amount a user sends out of their steem account.

The data leads me to guess that those huge outflows correlate to the price of STEEM dropping. Perhaps to sell for BTC in order cut their loses? I'd be interested to see the price action of BTC and STEEM on your net inflow-outflow chart.