If the person pulling the trailer is insured then normally her property damage liability insurance would cover the damages to your vehicle.

If the person driving the trailer was not insured, or if their insurance would not cover the trailer or objects that fell out of the trailer hitting your car and damaging it, then you would need to place a claim with your own physical damage coverages. Collision and comprehensive fall under this type of auto insurance.

If the claim is going under your insurance policy and not the other person's then you will need to report this incident to your insurance company and they will investigate your claim of what happened and the damage sustained to determine if it will covered by your collision or "other than collision", comprehensive, coverage.

It will likely matter what object it was that fell from the pickup and if your car and/or the truck was moving at the time accident. If it is determined that the damage is not covered by collision then it would probably be covered by your comprehensive coverage. With either type of coverage you will have to pay the deductible as part of the claim process.

In general a collision occurs when your vehicle collides with another object on or attached to the road or ground. Flying objects, such as birds and airborne debris, are not on or attached to the road or ground, and thus are not considered collisions. Many of these types of claims are paid under the comprehensive coverage of an auto insurance policy.