In general, the ISS altitude decreases gradually with time due to drag, and increases almost as a step function (in the time-scale of this graph) as the ISS is boosted back up to keep it within normal parameters.
However on or around January 27th 2015, there is a sudden loss of altitude of about 1.5km. Given the shape of the rest of this graph, I assume that this was not due to dynamic atmospheric conditions, but more likely a planned maneuver. If it was a planned maneuver, it appears to be in the wrong direction - it looks like the use of (precious) fuel to achieve a result that could have been achieved just by waiting 10 days or so.
Was this a planned maneuver or was it due to natural atmospheric conditions? If it was planned, what was its reason?