John Eric Lloyd was checking an oil leak at his family's plant hire business in Dolgellau, Gwynedd, in August 2015.
The arm of a hook loader - used to load and unload skips - fell on him, killing him instantly, the inquest heard.
His father, Alun Lloyd, said he "cannot fathom" why he did not prop it up. The hearing in Caernarfon continues.
Mr Lloyd's sister, Sharon, who witnessed the incident, described her brother as a "competent and confident" heavy plant operator and said she had told him to "get out of there" before it happened.
A 23-year-old man was crushed to death after a piece of heavy machinery fell on him, an inquest has heard.