An interesting find. I would imagine that the fact they all spiked around the same time might have more to do with the way Alexa measures traffic, or perhaps the way Alexa itself is promoted? Perhaps there was a surge in Alexa discussions at that time that led to more webmasters downloading the toolbar?
I had a look back through the TechCrunch and Problogger archives and I couldn't find anything really obvious that would account for the spike. Looks like early April 2006 to me. Strange indeed.
well i think at that interval alexa changed thier algorithm such that people who visit alexa will only be considered (as it is now) , at that time thier wouldnt be any such ranking algorithm may be i am wrong i got introduced to webmastering few months back