The truck driver lost control ,was on the wrong side of the road and hit the bus head on. The bus driver even had time to stop the bus and open the doors to let a few passengers get out before the inevitable impact.
The road was so narrow there was not room for 2 large vehicles to pass.
This a problem in 'Rural France ' there has been far less investment in new roads than in the UK, many routes are virtually unchanged from the 1800s, and not suitable for modern traffic. The 'NIMBY' culture in France is far stronger than the UK and there will be no change anytime soon.