This attack leading to the wrecking of the Impala was performed by a black-eyed demon on the Winchester family. It is not clear whether the demon was affiliated with Azazel, following his orders, or not.
Background
After Azazel fled, Sam, Dean and John head out of the cabin and onto the road to discuss their plans in hunting down the yellow-eyed demon.[1]
Battle
The conflict was a rather quick assault on the Winchester family. As the three were cruising, they were plowed by a massive truck driven by a black-eyed demon possessing a male human.[1] The impact was such that Dean, who already suffered fatal wounds from Azazel, spiraled into critical condition after the wreck, while John, who was closest to the impact, lost consciousness, and would later be discovered to have injured his arm. Sam was heavily wounded, but later managed to achieve consciousness as the possessed truck driver inspected the wreck.[2]
As the demon pulled apart the Impala's door, Sam readied the Colt and threatened to shoot the demon. The demon was convinced Sam was bluffing, as it knew that the Winchesters were saving it for Azazel. Sam cocked the Colt and aimed at the demon, who was forced to leave his host.
Aftermath
The wreck would later cause Dean to die a biological death. This in turn, would force John to make a deal with Azazel. John would trade his life and the Colt in exchange for Dean's resurrection.