Bumped for paying low price

Was on a flight from Denver to DC last week. Checked in as soon as it was available and was given a boarding pass and seat number. Get to gate ahead of time, boarding is ongoing. When I try to scan my valid boarding pass the agent tells me I actually don’t have a seat as the flight is overbooked. My seat was bumped as I “paid the least amount for my ticket” compared to the rest on the flight. Some people ended up not showing up so I was still able to fly, but any advice for if this happens again? Not understanding why me getting a deal means that I’m sold something that doesn’t exist. Is this in their fine print somewhere?

Edit: I booked through Frontier and not a 3rd party. Saw the price drop about 40$ from a few days before and said why not!