If we're talking "top 5" in the context of graduate programs for a given discipline it's likely not as rare as you'd think.

I can see the PhD glut and general awfulness of academia (do postdocs and adjunct until a tenure-track opens up) pushing people from even high ranking programs into teaching at a CC.

This is especially true if they're married or have an SO: It's hard to convince your partner to uproot their lives so you can work at some tenure track position at some no-name college in the middle of nowhere... or forgo tenure and get paid slightly less and have the option to actually live near family/friends/partner's career.

Warning N = 1, but I went to CC and there were plenty of Top 5 graduates working there. There are lots of reasons someone with a PhD from a top program might want to work at a community college. Passion for teaching, proximity to family, and much more flexible schedules are all reasons to choose to work at a CC over a 4 year.