Is the questioner liable for an accident that occurred as a result of his action, and is he sinful for not taking the other party's information, and what should he do regarding the accident and the unknown individuals?

You are not liable because your slowing down was not the direct cause of the car overturning, and the fault lies with the driver behind you. Even assuming you suddenly slowed down, liability falls on the direct actor, not the indirect cause, according to the legal maxim: "When a direct actor and an indirect cause combine, liability is on the direct actor." However, if the direct actor cannot be held liable, then liability falls on the indirect cause.