If you can have too much demand for something, you can also have not enough resources to handle that demand. What causes burnout is not having enough resources or reserves to draw from when you need it. Those resources may include control of the situation, time off to recover and refresh, skill, knowledge, assistance, or even attitude.
If you have intense demand over an extended period of time with diminished resources, you run the risk of stress, overload and increasing burnout symptoms. Leaders become demotivated, dispirited and even depressed. Periods of intensity require adequate resources to meet the demand.