With regard to your observation about the driftwood coming and going on the beaches, i checked out something about the currents running up the coast of Taiwan.

Basically, there is one main one, known as the Black Tide in Taiwan (hei chao, or the Kuroshio current. It's one of the world's major ocean systems (called gyres) and travels in a circular path - we being on the western boundary of it. The direction is always flowing to the north past Taiwan.

However, there are these things called 'eddies' which are supposedly like the 'storms of the ocean'.
They seem to be quite large - several hundred km's across, and apparently can hold their form for several months and have a significant impact on the biology of the area they are affecting.

As for the driftwood - they could possibly explain why there was a period where more seemed to be washing up.
Of course the change in the monsoon direction would seem to be a pretty plausible explanation too.

Additionally, from the literature, there seems to be some interaction with ppe sea current which i don't quite understand, but it supposedly injects into the black tide during the south west monsoon. This interaction is also a contribuiting factor to the formation of eddies.