In addition to the factors Anda mentioned, I would like to add one from Finland. The latest magazine of the national teacher association had an article on this topic. One factor behind the burnout was the decentralised administration. In addition to the actual teaching the schools have to take care of thei own working conditions within limited budget. This means one additional task for the teachers who in many cases feel they do not have a mandate for this task.
On the other hand the teachers indicate that one way to struggle against the burnout is a possibility for in-service training to obtain the new competences in meeting the wider task and the workload. From this perspective the schools have not been able to provide equal opportunities for staff development programmes.