This is an intended feature of the current interest rate model; it generates reserves when above 50% utilization, and slightly churns reserves when below 50% utilization (though in a way that should, in theory, grow the market size).
You can see the model in this thread from the launch of the market.
It should be noted that the interest rate model is only one of two systems that can increase and decrease reserves; the liquidation process also increases and decreases reserves.
In aggregate, the reserves of the market have been increasing in the aggregate since launch, though this should be continuosly monitored (and your participation here is helpful!)