The nine-metre trailer used by Alister Clark, 57, had a series of defects and was unroadworthy, a court heard.
After uncoupling from the tractor on a bend, it slid across the road and Stuart Reid, 34, did not have time to avoid driving his pick-up truck under the trailer. The roof of his vehicle was torn off and he suffered extensive head injuries and died at the scene.
Clark's lawyer told relatives of Mr Reid at the High Court in Edinburgh yesterday that the accused "cannot find words that adequately express how much he regrets the distress he has caused them".
Clark, a first offender, of Station Cottages, Ballindalloch, Moray, pleaded guilty to causing the death of Mr Reid, a tree surgeon, of Dulnain Bridge, near Grantown on Spey, Moray, by dangerous driving on 19 October, 2008.
The advocate-depute, Leanne Cross, said the accident happened on a Sunday morning on the A95 near Ballindalloch. Clark was driving a tractor and towing the unladen flat-bed trailer to a farm at Nethybridge. Mr Reid was heading in his silver Mitsubishi truck to a golf outing in Huntly.
The vehicles approached a bend from opposite directions, the tractor going uphill.
Miss Cross said a trailer could not be connected directly to a tractor, but required a "dolly converter" to act as a bridge.
"As the accused negotiated the bend, the trailer separated from the dolly … and slid across the carriageway and collided with Stuart Reid's vehicle. Owing to the height difference, the front of the car effectively travelled under the trailer," she added.
"The deceased was unable to take evasive action in terms of his vehicle, but appears to have leaned away in an effort to avoid being struck – without success.
"The entire windscreen smashed and the top and offside of the vehicle were removed by the force of the trailer."
Staff from Ballindalloch filling station heard the collision and went outside, but realised immediately the severity of the situation and returned to contact the emergency services. Passing motorists stopped to help, but it was clear that Mr Reid was dead. He had suffered severe and extensive head injuries.
Miss Cross said the dolly converter and the trailer were found to be in a very poor condition, with numerous defects. In particular, the braking system of the trailer was not working and, in any event, had not been connected.
"Had the dolly and/or trailer been stopped by the police, an immediate prohibition would have been placed on both – prohibiting them from travelling on a public road in their current condition," said Miss Cross.
"As there were no air brakes on the trailer, it was free to travel when it became detached, with no emergency brake function available to draw it to a halt.
"Experts cannot say exactly where the trailer would have stopped had the brakes been in operation. They are, however, of the view that the trailer would have immediately started to slow, offering the deceased more time to react to events."
It was thought the dolly and trailer might have been miscoupled, and Clark admitted to police that he had not checked the connections.
Miss Cross said Mr Reid's parents and his long-term partner, Jennifer Petrie, had been devastated by his loss.
Clark will be sentenced later this month.