Around May and a bit before then it was so decided that whereas the people in charge of Monero don't currently like botnets more than the effect of not having them mine Monero, the method in which mining Monero was changed by the people that have the ability to change the source code of Monero, in order to discard the old botnets during some likely routine hard forking event.
In reflection of this change becoming a reality, the difficulty algorithm adjusted itself to reflect this action. As the hashrate is derived from difficulty, the graph that displays hash rate was left with a rather abrupt and jagged looking cliff. Whether or not it was a mistake is a matter of speculation likely for some time to come, though it was unquestionably a major event.
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