There's nothing preventing someone from filing suit in a situation like this. Sometimes attorneys will sue marginal parties just to be sure they are covering all bases. The more important question is whether they will be successful in the suit which will require that they prove some negligence on your part. Your insurance company will hire a lawyer to defend you and that lawyer will advise you on all of the considerations in your case.
Let your insurance company defend the claim. That's what you pay your premium for. The car that rear ended you likely doesn't have insurance coverage so bringing a claim against you is the other cars only option.
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Report this to your insurance company ASAP. They should handle it for you.
The above is general information only and is not legal advice. The information provided does not form an attorney-client relationship, and should not be relied upon to take or refrain from taking any action. I am not your attorney until we sign a retainer agreement.