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I would suggest you to check this article for better understanding my theory–>> Here
As we know , The 9 tails attack on Konoha made a devastating effect. The loss was certainly huge. And all this happened in the absence of the 3 Legendary Sannin. What if the Sannin were present during the attack?
So, in the original story, when Kushina gave birth to Naruto , it was obvious that the seal holding Kurama would be weakened as it earlier happened with Mito Uzumaki. Due to this very reason, all the precautions were taken and the delivery was done on outskirts of the village. The place was obviously secured.
Obito took the advantage by taking Kurama under his control with his sharingan. Kurama just broke over the place and caused a shit load of destruction. At the time of attack, the only noticable and strong shinobis were the Lord 3rd and 4th. All the other Shinobis tried their best trying to attack Kurama but he was literally a beast.