He had probably already started the concerto when he heard the sad news of the death of a girl called Manon Gropius , the daughter of Alma Mahler ( who had once been Gustav Mahler 's wife ) and Walter Gropius .
The event that spurred him into writing was the death by polio of 18-year-old Manon Gropius , the daughter of Alma Mahler ( once Gustav Mahler 's wife ) and Walter Gropius .