According to the article:
"The recent spate of arrivals has drawn attention, though such discoveries are not out of the ordinary: Dozens of such wrecked boats drift toward Japan every year."
"So far this year, 34 mystery boats have drifted over, including the 11 found between late October and November. Last year, Japan found 65 of them. In 2013, there were 80, according to the coast guard."
"The number tends to rise during the fall and winter season because of prevailing winds from the northwest, Hiroto said."
I doubt anyone would care to expend the time and resources to try and figure out why some Junkets went rogue centuries ago but it seems self evident the reason they are happening today is due to northwest winds. Poor people go out in crappy crafts to either fish or escape, something disables the craft and they drift toward Japan. Doesn't seem like rocket science.