Sainz crashed into the side of Lance Stroll's Williams while exiting the pit lane on lap 11. The collision put both drivers out of the race and triggered a safety car.
"I knew it would be very tight at the end of the pits with Lance," the Toro Rosso driver explained. "I tried to take the corner around the inside, and I didn't lock up the tires.
"He must have not seen me and turned it straight into me and we crashed," he continued. "You are allowed to turn in. But when you have someone inside, you have to leave at least one car's margin in case there's someone there.
"He simply didn't see me, and wasn't expecting me to be there," Sainz insisted. "He chose to take the risk and assume no one was there."
Naturally, Stroll entirely disagreed with Sainz' account of the incident.
"I saw Sainz coming out of the pitlane," he revealed. "I was 50 or 60 meters in front of him in the braking zone, and was already turning in. He drove into my side. There is not much else to say. I have just seen the video and it was ridiculous."
The race stewards reviewed the video evidence and heard from both drivers before agreeing with Stroll and penalizing Sainz. They also reviewed the video evidence which confirmed that Stroll was on the normal racing line as Sainz exited the pit lane.
The stewards determined that Stroll had been on the normal racing line and that Sainz "made a very optimistic attempt to pass". They concluded that the Spaniard "was predominately to blame for causing the collision in violation of Article 27.4 of the FIA Formula One Sporting Regulations."
As well as the Sochi grid drop, Sainz was also handed two penalty points which makes a total of seven over the past 12 months. The penalty is not subject to an appeal by either Sainz or his team.