Brand new baseballs are put into a machine, located in the umpire's room, and mixed with sand before use. This is done to make baseballs a little less slippery and is covered in section 3.01 of the rulebook (Umpires will confirm that baseballs are properly mixed with a special sand for an unspecified amount of time).
According to a Nikkan Sports report, when Yu Darvish pitched on the 15th, he turned to an ump in the second inning and asked that the baseballs be put through the machines for a longer amount of time. The request was turned down. And then in the third inning, he uncorked a wild pitch that ended his consecutive scoreless innings streak.
The special sand that Mizuno provides umpires with is made out of sand from Kyoto, dirt from Kagoshima, and a chemical compound all mixed together. Umpires have been suggesting that Mizuno increase the amount of black dirt they use in order to make the balls less slippery. The NPB is now getting into it as well and as of yesterday, was scheduled to meet with Mizuno today and discuss what can be done. And it seems for now, there's a good chance the sand will be improved.