There was a post in the blockchain.info thread stating that the pool charts were a bit broken. Generally any time you notice ANY strangeness in a hash rate chart, it's related to a problem with the chart generation or a DDoS.
Thats kinda a bad dichotomy.
General 1: "Any time this light flashes either we are under imminent nuclear attack and should launch our nuclear arsenal immediately, OR there is a short in the wire in which case launching would start the nuclear war we thought had already begun"
Light: <flash flash>
Well, those really are the two things that make these types of charts act strange. And in the case of a DDoS, miners tend to post a couple dozen threads about three seconds after it starts, so they're not too hard to spot.
Although I did miss one that affect charts using Pool APIs to determine things like hash rate: The pool API reporting bad info (one pool becomes way larger on the chart than they should) or no info at all (they disappear from the chart and Unknown grows to compensate). Blockchain.info doesn't use that method though, so its not a problem.