I rear-ended another vehicle that was towing a trailer and the driver injured his back

He may be. If the claim were to go to court, it would mean proving a causal relationship between the use of the vehicle and the injury. Courts across the country have given different decisions, but a general thread is that there must be a causal relationship between the vehicle and the injury. And, as the claimant says, the injury would not have occurred but for the accident.

The facts would be up to the court to evaluate, including any relevant medical information that either substantiates or refutes the claim.

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