I think it's two things:
1. They re-engaged a lot of their P1s by embracing the station's culture. Bringing back Kevin in the afternoons and Sluggo in middays, expanding and growing the Klein & Ally show, and bringing back the original imaging and sound design that the station had in its heyday. It's comforting to hear something familiar with so much change and economic uncertainty.
2. Nostalgia is very in right now with younger listeners. Just think about how "Iris" by Goo Goo Dolls is a top viral song. Deftones, System of a Down, Sublime, Nirvana, Korn have also found some vitality on social media platforms. This positions KROQ perfectly because they are the modern rock station that broke many of these bands, and clearly it's reflecting in the numbers.