If we look a the current scenario on https://www.blocktrail.com/BTC, block 390470 to 390474, i.e. 5 blocks, have been generated within 25 minutes, i.e. 5 minutes per block, whereas 10 minutes is the average. This is nothing unusual. Sometimes the luck favors, sometimes it does not. On the long run average remains 10 min dues to difficulty adjustment.
Difficulty doesn't change when the hashrate changes till the next adjustment period. In the long run, Bitcoin would likely not be reaching 10 minutes per block.There should be only 24 blocks generated during the past 4 hours if varience doesn't exist but it does. However, more than twice was generated. It is possible that the entire network was extremely lucky or someone added a massive amount of hashrate to Bitcoin. This warning exist to warn merchant of a possible attack on the network (ie. 51% attack).
I would believe that the network was extremely lucky and an amount of hashrate was added.
