We have all seen that in last chapter juubi seen Rikudo in Naruto after that Juubi formed some hand seals...
actually we can see that it is not some seal..Juubi is just clapped it's hands.....In Naruto it is not only juubi but also other characters also do this
First one is Hashirama senju. He do that clap seal when going through sage mode or shinnensenju.
Next one is Jirayia see this image.
He claps hands when he go through sennin mode
Next one is Naruto see this image
This is when naruto dispels illusion of itachi
So my theory is that the juubi is undergoing some kind of transformation or simply bring a lot of chakra. Since juubi's chakra is natural energy.It will not go sage mode because there is no need to it. So my conclusion is that juubi is going the final transformation or bring more chakra.