It's quite normal. Bitcoin's price was too low to mine for most of miners, When the price went up home miners etc started their rigs.
It wasn't the first time we've seen 400+ Phs (according to blockchain.info's graph).
I was about to say that, but I didn't confirm on the graphs... also though so
And yeah, it's just basically people turning on their miners because of the price increase. The wrong time to turn them on, IMO
We have reached it before, but that sudden increase and then it disappeared makes me think it could be next generation hardware testing. That's just speculation of course
My thoughts also
But I don't have any numbers or proof about that... Maybe KnC and their 16nm chips?