Carelessness, wind and rain.
Humans are careless, so leave all manner of plastic outside in their yards. A gust of wind catches the plastic and blows it onto a nearby road. It rains, and the plastic is washed into the gutter and stormwater system, where it travels through drains, culverts, creeks, rivers and ultimately out to sea.
That's why pollution at outflows is always worst after a storm... where wind and rain go hand in hand.
Commercial and industrial premises are just as bad as residential ones — if not worse — but the cause is still the same: Carelessness, wind and rain.
Of course we can (if we want) specifically blame winds for toppling garbage bins set out for collection, and animals for digging through trash, and the homeless for rummaging through dumpsters and leaving the lids open, and kids for unwrapping their toys outside, and transport companies for poorly wrapped pallets, and construction companies for a plethora of loose materials on building sites, and children for posting 'lost dog' signs on electricity poles, and nationalists for flying flags, and Christians for decorating outdoor Christmas trees, and any of a million other groups of people for putting/leaving lightweight plastics outside where the elements can relocate them... but the root cause is still the same for all of them — so it makes more sense to focus on the root cause.
Unfortunately, having garbage that 'disappears on its own accord' is probably seen as a good thing by the 'average' person, not a bad thing, so I doubt you can change people's behaviour enough to make a measurable difference. As long as 'trash' is something people 'put outside' and ultimately 'someone' or 'something' makes it 'disappear', this problem will persist.
"Out of sight, out of mind" is a powerful enemy.