-- Early in the pandemic, travel was way down. Rental car companies unloaded a lot of their fleet that were sitting idle. Now that demand is back up, they are back on the market for vehicles.
-- Semiconductor shortages: automobile manufacturers cut their orders in early 2020 anticipating a decline in demand. That decline never happened, so they're now now able to obtain sufficient chips for new vehicle manufacturing. Thus a decline in new car availability that might push some car-buyers onto the secondary market.
Both. New cars are very expensive and most people can't afford them. The used car market is nearly 3x the size of the new car market (in cars sold, not $).
Also, as cars have become more reliable many who would otherwise buy new are buying lightly used off-lease cars (i.e. certified pre-owned). Many people understand the instant depreciation hit that comes with a new car, and want to avoid it.
Add to all that there is currently a supply chain issue for critical parts needed for new cars.