What causes that 'fresh cut grass' smell? Trauma, that's what. It's the smell of chemical defenses and first aid. The fresh, 'green' scent of a just-mowed lawn is the lawn trying to save itself from the injury you just inflicted.
Leafy plants release a number of volatile organic compounds called green leaf volatiles. When the plants are injured, whether through animals grazing on them, you cutting or mowing them, or even just unintentionally rough handling, these emissions increase like crazy.