Two fundamental trends are driving this influx of foreign blood into Japanese sumo. The first is that the sumo stables are having a harder and harder time finding Japan-born youngsters to enter the demanding, grueling world of sumo, where all the wreslters in a stable live together and new wrestlers are put to work cleaning toilets and doing the other least desirable jobs there. Unable to find new blood at home, sumo stables are looking beyond Japan's shores for talent.
The second trend is the growing popularity of sumo worldwide, which is creating a larger and larger "pool" of wrestlers from which Japanese stables can recruit. In decades past, Hawaii was the only foreign source of wrestlers.
In fact, the pioneers of foreign sumo wrestlers were all from Hawaii: Jesse "Takamiya" Kuhaulua was the first foreigner to enter professional Japanese sumo, in 1964; the massive Salevaa "Konishiki" Atisanoe was the first non-Japanese sumo wrestler to reach the rank of ozeki (champion); and Chad Rowan, better known as Akebono, became sumo's first non-Japanese to achieve the highest rank in sumo, yokozuna (grand champion), in 1993.
There's a PBS documentary about the "globalization" of sumo called Sumo East and West.