Chapter 936: Kamui
Fifty kilometers outside Konoha.
Obito stood on the head of the Eight-Tails Gyūki, staring coldly and angrily at the giant fire mushroom that was gradually dissipating in front of him.
"Kyubi Jinchuriki, I underestimated your resolve."
After Obito 'swallowed' Kushina's distress earlier, he controlled Gyuki to launch a frenzied attack.
Following a fierce, earth-shaking physical brawl between the two beasts, they moved on to magical attacks, and finally, as it should be by rights, turn on the 'nuclear' attack.
As a result, Isobu chose to endure a direct hit from Gyuki's Bijudama, and took the opportunity to deliberately misfire his own Bijudama towards Konoha!
Based on Obito's understanding of Isobu's IQ, it is absolutely impossible for him to think of this, so the answer is obvious.
This is Kushina's plan!
She knows that she and Isobu together couldn't defeat him, so she intentionally had Isobu direct his Bijudama towards Konoha to alert Minato!