Why are they back now? Because they WANT to get exposed to radiation. Why? Because they believe that would get them a baby boy. It's got to be another "baseless rumor"; high-IQ and rich Chinese wouldn't do such a silly thing as trying to be intentionally exposed to artificial radiation hoping that would get them a boy, would they?
From Shukan Post article (July 22/29 issue) as appeared in Iza:
若い中国人カップルの間で「訪日ツアー」がブームだそうだ。
We hear that tours to Japan are becoming very popular among young Chinese couples.
震災直後、中国政府の帰国勧告を受けて各地の空港が中国人の帰国ラッシュで大混雑したことは記憶に新しい。そんな放射能パニックに陥った中国人が戻ってきたのかと思えば、どうも歓迎できない動機がありそうである。
We still remember the local airports crowded with Chinese citizens returning home upon the recommendation from their government. Why would the Chinese, who fled in a radiation panic, come back? There seems to be a reason behind it which we cannot totally be happy about.
「被曝によって男児が生まれる確率が高まる、というドイツの研究が中国紙で報じられたからです」（中国在住の日本人ジャーナリスト）
"It's because a German study was reported in a Chinese paper that radiation exposure increase the chance of conceiving a baby boy", says a China-based Japanese journalist.
『北京晩報』は6月8日付で、〈放射線で男の赤ちゃんが増える〉と題した記事を掲載。核実験が頻繁に行なわれた1960～1970年代にかけての欧州・米国や、チェルノブイリ原発事故の2年後のベラルーシ、ドイツとスイスの原発周辺の地域でも男児の出生比率が高くなったという怪しげな研究発表を報じたのである。
On June 8, Beijing Evening News carried the article titled "Radiation increases male babies". The article cited a dubious study that the ratio of male babies increased in Europe and the US during 1960s and 1970s when nuclear testing was done frequently, in Beralus 2 years after the Chernobyl accident, and in Germany and Switzerland in areas near nuclear power plants.
中国のネットユーザーは記事に即反応。中国版ツイッター『微博』には〈男の子を授かりたいなら日本に行け〉〈すぐに日本行きの航空券を買いに行く〉〈旅行会社は男子懐妊ツアーを組んでくれ〉などの書き込みが相次ぎ、ついには〈訪日旅行を盛り上げるために小日本が考えたデマじゃないか？〉という陰謀論まで登場した。
Net-savvy Chinese immediately responded to the article. The Chinese version of Twitter [I suppose it has nothing to do with Twitter but a Twitter-like service in China] was flooded with messages like "If you want a boy, go to Japan", "I'm going to buy a airline ticket to Japan right now", "Travel agencies should arrange "Boy Conception Tours"." And conspiracy theories like "It must be the false information concocted by "small" Japanese to revive the tourism from China."
瀋陽の日本総領事館に問い合わせると、4月にはゼロだった訪日ビザの発給数は、5月に23人、6月に1900人と大幅に増えている。
We inquired the Japanese Consul in Shenyang. There was no issuance of visitor's visa to Japan, and only 23 in May. But in June the number exploded to 1,900.
中国人団体を取り扱う日本の旅行会社の添乗員が苦笑交じりに語る。「九州の阿蘇山を案内した時、新婚の中国人カップルから、“放射線量はどのくらいか”と尋ねられました。“何も心配いりませんよ”というと、男の赤ちゃんが欲しかったそうで肩を落としていました」
A tour guide at a Japanese travel agency that cater to group tours from China says with a wry smile, "When I guided the Mount Aso in Kyushu, a newly wed Chinese couple asked me, 'How high is the radiation level?" When I told them there was nothing to worry about there, they were very disappointed. They said they wanted to have a baby boy."
This particular couple probably didn't bother to look at the map of "small Japan". The island of Kyushu is as far away as you can get from Fukushima I Nuke Plant. They should be enjoying the hot spring resort near the plant with the "Genpatsu Gypsy" workers (read the Guardian report).
The Japanese version of the same "baseless rumor" about radiation and conception is that if you work in a nuclear power plant you are more likely to get a baby girl, not boy. Maybe it's different with Chinese.