The hash rate is computed based upon the daily average of found blocks and the current difficulty. It is not directly reported. So, the variance you see probably has more to do with the random luck of finding coins rather than the actual computing power of the network. So, if a bunch of blocks are found in a short period of time, the algorithm infers there was a large hash rate increase, even though the actual hash rate may have not changed. As the rate of finding blocks returns to normal, the inferred hash rate decreases. Since it is averaged over time, you see these relatively smooth rises and falls.

In short, the swings mean absolutely nothing other than pure variance. The longer term trend of course does indicate the increase in total network has rate.