Who knows how water may have seeped in. Owner could have even left his window open several different times when it rained. Once water gets in those rockers, and doesn't drain...it will never dry out.
I've even seen salt spray eat through the bottom of those rockers when left untreated, if you spend several hours a week driving on winter treated roads. Those older steel ford f-150's were noted for the same area getting chewed out. At any rate when you live in the rust belt, gen1 tacoma's were notorious from rotting from the inside out...the gen2 not as much, but with both usually when you see it, its too late to do much about it, other than seeing a body shop.