Answer:
The highwayman's anger was caused by the discovery of the death of Bess, the girl he loved.
Step-by-step explanation:
The poem "The Highwayman" by Alfred Noyes tells the story of an unnamed highwayman falling in love with the daughter of an innkeeper. But the betrayal of Tim, the ostler, led Bess, the girl to sacrifice her life to save the highwayman from getting killed by the authorities.
The lines "back he spurred like a madman, shouting a curse to the sky" shows his anger at discovering that Bess had been killed while trying to save him. This was her way of making sure that he did not get captured and then tortured by the authorities, thereby showing her love for him. Thus, the anger in him was a result of knowing Bess, his lover, had died at the hands of the authorities who were waiting for him.