These kinds of 'reboosts' are entirely normal. Eventually the station will have its own propulsion system to compensate for orbital decay, but until the facility has a propulsion module, it's going to need occasional lifts from the shuttle. The size, shape, and orientation of the space station is critical. When the solar panels are added, for example, it can dramatically increase the frontal area of the ISS and its aerodynamic drag. Solar activity is important, too.
Orbital decay is much greater when the sunspot number is high.
The solar cycle has a big effect on the thermosphere where satellite drag takes place, agreed David Hathaway. That's high, but not nearly as high as the temperature during Solar Max. The density of the thermosphere can soar by a factor of 50 during solar maximum, with a commensurate increase in atmospheric drag on satellites. In 1996, near solar minimum, the EUV Sun was nearly featureless. Now, near the peak of the cycle, the Sun is dotted by fiery regions of hot gas trapped in magnetic fields above sunspots and plages.