This is a theory of Rikudou's last day i believe to, our opinions can be different.
Rikudou was continously getting older and older...weaker and weaker. He was afraid that one day the seal will get broken and he will not be able to stop the ten tails, so he prepared very well.
The senju clan was original to Uzumaki clan, which had longevity, so I believe Senju had it also. I believe Rikudou hasn't died due to old age, but due to extraction of tailed beast and exhaustion.
We all remember when Naruto extracted Kurama's chakra, he still had it and was able to do a tbb 4 times bigger than his own body.
So Rikudou used a seal, the "gates fall jutsu"(as i call it), seen here:
with it he was able to get Jubi's chakra, but Jubi was so strong that he left Rikudou's body and wanted to terrorize the world again.
Rikudou had to use a lot of Jubi's chakra to create a moon and seal Jubi's body there. He split the rest of chakra with izanagi(coat) into 9 parts, the tailed beasts. He knew that it would be against the nature not to split the whole chakra, so he created the tailed beasts young and they will continously age and regenerate the used chakra back.
With his remaining eye, he created 2 humans, his "sons". The first was the Uchiha ancestor and the second, the Senju ancestor. And we all know that the elder one had Rikudou's eyes(probably a weaker rinnegan) and the younger one Rikudou's body(senju strong body(as seen by Obito, he got hit by rasengan and was perfectly OK))
Well, this is my theory, thanks for reading