How did a whole bucket of leaves and dirt get behind the kick panels in my classic

[Bashful Bondage Bunny]

chevy? I removed the kick panels and the vent-mechanism thing and reached inside... and i found leaves and dirt! They had been there so long they had turned to black dirt. I pushed the stuff around for a while trying to figure out where it came in, but it didn't come out anywhere and fall on the ground under the truck or something... and i think i pushed some of it deeper into the shell of the truck to be stuck there forever... I ended up sucking it out with a shop vac... but where is the stuff coming in? What do I have to take off to access that space besides the vent itself? This is a 66 chevy with the knobs you pull to open vents behind the kick panels. And how do i keep debris from getting back in there?
i am clueless... it wasn't like the dust and crap that builds up everywhere else... it was like the soil you can buy in a bag... and it filled a 2g bucket. how did that happen?