Sonakshi and Rohit seem to be linked to the same accident that happened in their past. We know it is an accident since Sonakshi's ex mentioned a car accident associated with 26 September and same day Rohit operated on someone who succumbed to their injuries.
I think Sonakshi (or someone from her family) was the one who hit that patient with her car. Since she didn't defend herself when her ex accused her of it. And the patient was admitted to Rohit's hospital but he couldn't save him/her.
Now I have theories about who slapped him.
1. The story maybe like 'Dil Ka Rishta' where Arjun crashes into Ash and her hubby's car. Hubby dies, Ash loses memory. Later falls in love with Arjun but he cannot tell her he was responsible for her husband's death.
Sonakshi's ex mentioned an accident, I think it is the same one in which the patient who died in the OT was injured and Rohit couldn't save him/her.
2. His ex slapped him after saying 'don't even call me' and then while driving back she had an accident in which Sonakshi's car hit her.
3. The person injured in the accident was Rohit's ex's relative/parent and she left him coz he couldn't save them. This would explain why Rohit keeps calling on an out-of-reach number. Coz if the ex is dead, whom is he still trying to call?
I think they will reveal it on 26 September only. And by then the leads will be in love and then they will bring up a separation track because of this.