This is quite interesting... When you look at the difficulty (https://bitcoinwisdom.com/bitcoin/difficulty):

Why does the hash rate go down before the difficulty goes up? I never thought about this - maybe it's a typical newbie question.

Before the last difficulty jump (Sept. 13th?) this hash rate drop was not that high. I wonder if some folks try to manipulate the difficulty by switching on some TBs of hashing power at the right moment?
I don't know how the difficulty steps are calculated. I hope that it's not possible to manipulate them in this way... (Or: Finally, it's not a manipulation, because this TBs of more hashing power really exists, even when they are switched on only temporarily)