The only explanation is organized, industrial-scale dumping from coastal countries (particularly Indonesia and Southeast Asia) that lack landfill and reclamation infrastructure.
At any rate, it would great if this were actually the case, because then it would be easier to bottleneck and corral this trash. But, I don't think this is the case.
The Amazon is dumping 450,000,000 cu ft per minute into the ocean on average, 1/10th of a garbage truck in such a vast quantity of brown water filled with floating crap seems very reasonable. Further, this is an average the bulk is probably dumped during floods when the flow is higher and plastic floats from roads and whatnot into over flowing drainage systems.
OK, fine, I'll do the math for you.
Here's a list of discharge rates for most of those rivers: https://en.wikipedia.org/wiki/List_of_rivers_by_discharge add in the Hai river at 717 m^3/s and the total discharge volume of those 10 rivers together adds up to 123,500 m^3/s, that's 7.4 million cubic meters per minute.
What do you think a few cubic meters of plastic diluted into over 7 million cubic meters looks like? It looks like a dilution level of around 1 part per million. Is that representative of the hugely polluted outflows of these rivers? It certainly seems like it to me.