If you read the box below the chart it spells it out pretty clearly:


What's going on with the difficulty? At block 135,000 a scheduled hardfork happened. Due to a bug in the new difficulty algorithm, the network difficulty first dropped to a very low number, then started targeting a block time of approximately 30 minutes (instead of the correct 10 minutes). This bug is being fixed by a new hardfork at block 139,000. It will take a few days before block times go back to normal (source). Meanwhile, ASICs have started mining, causing the difficulty to rise further.