The German pulled off the track close to the pit lane exit having just made his second pit stop on lap 23 of Sunday's race. As fire spread rapidly through his car, Heidfeld was forced to make a hasty retreat from the cockpit of the R31, with marshals then fighting to contain the blaze.
This they eventually did, but only after a small explosion showered the track in debris. Allison has now revealed that the combination of a prolonged pit stop, a cracked exhaust and a different engine mapping caused the drama, which has resulted in the chassis being declared a write-off.
"As with most accidents, several incidents combined to cause the fire that Nick suffered in Hungary," Allison explained.
"First of all, we ran a slightly different engine mapping strategy in qualifying, which produced hotter than normal exhausts. We believe that this elevated temperature and caused a preliminary crack in the exhaust pipe.
"We presume that the crack then propagated during the laps to the pit stop - this was not evident to us as we believe that the failure occurred upstream of the place where we have a temperature sensor. We believe that Nick then came in with a partially failed exhaust.
"This pit stop took longer than normal, the engine was left at high rpm for 6.3 sec, waiting for the tyre change to be completed.
"Under these conditions, a lot of excess fuel always ends up in the exhausts and their temperature rises at around 100°C/sec. This temperature rise was enough to finish off the partially failed pipe and to start a moderate fire under the bodywork."
Regarding the explosion, Allison said that "this was caused by the air bottle which supplies the air valves in the engine. It has overheated in the fire and failed."
The team have already taken their findings to the FIA, with Allison saying they are doing everything to ensure that the incident will be a one-off.
"The incident was highly undesirable, as it has caused us to write off a chassis," he added.
"We will take steps prior to the next race to reduce the likelihood of a further fire and to ensure that the air bottle cannot overheat.
"We are in touch with the FIA both to provide them with a full report of the incident and also to explain to them the actions we are taking to prevent a re-occurrence."