The investigation has said that "the driver was overstressed. He could not find a break and was assigned on double duty," Road Transport and Highways Minister Oscar Fernandes said.
"There were fire crackers in the Bangalore-Hyderabad bus as well," he added.
In the first accident on October 30, 45 passengers were burnt alive as the Volvo bus they were travelling from Bangalore to Hyderabad burst into flames after its fuel tank caught fire on hitting a culvert at Mahboobnagar in Andhra Pradesh.
The bus might have caught fire due to a spark and fire crackers might have exploded, the minister said.
In the second incident on November 14, seven passengers were killed and 40 injured when a Mumbai-bound bus from Bangalore caught fire after hitting a road median at Haveri in Karnataka.
The causes behind the second accident are being ascertained, Fernandes said.
Meanwhile, a preliminary report by Accident Data Analysis Centre of National Automotive Testing and R&D Infrastructure Project has said that the buses had wooden flooring and inflammable seats and the speed limit was also high, according to sources.
The minister said he would personally monitor accident cases and ensure measures to prevent road accidents.
Last month, a Parliamentary panel had said it would seek a report from the road transport and highways ministry on the recent Volvo bus accidents in Andhra Pradesh and Karnataka.
Naidu sails on the BJP wave; to join NDA
Govt to fast track expressway projects
IN PHOTOS: BJP stalwarts attend Raje's grand swearing-in
GM recalled 1.6 lakh vehicles due to emission issue: Govt
Vasundhara Raje sworn-in as Rajasthan CM