This phrase exaggerates a high level of activity for dramatic effect. She's buying clothes like it's going out of fashion.
Pro Tip 2/3
Often Temporary
Used to imply the behavior is not sustainable long-term. He's playing video games like it's going out of fashion.
Pro Tip 3/3
Not Literal
Doesn't mean the activity will actually become unfashionable, but suggests urgency or frenzy. They're using up their phone data like it's going out of fashion.