Shares in the Japanese company's mobile telecommunications unit plummeted 15% in Tokyo on Wednesday, their first day of trading. The stunning plunge wiped out billions of dollars in market value.

The business is one of Japan's biggest wireless carriers and has provided the foundation for SoftBank CEO Masa Son's vast tech empire. By listing a big chunk of it, SoftBank Group ( SFTBY ) raised about $23.5 billion.

That makes it Japan's biggest ever stock float — and the world's second largest after Alibaba's ( BABA ) $25 billion listing in New York in 2014.

Strong demand from investors for a piece of the mobile business had prompted SoftBank to increase the number of shares it was selling. But it went public at a tough time for stocks: Japan's benchmark Nikkei index has fallen 7% since the start of December and is down about 14% from its recent high in early October.

The CEO of SoftBank's mobile division, Ken Miyauchi, blamed the disappointing debut on poor market conditions. At a news conference after the market closed, he said that SoftBank had originally planned the IPO for September or October, before delaying it until December.

Fears of a price war

Japanese mom-and-pop investors, who bought up most of the shares in the IPO, were dumping the stock on the first day partly over fears of a damaging price war, according to analysts.

Japan's biggest wireless carrier, NTT Docomo ( DCMYY ), will slash the prices of its cellphone plans by as much as 40% next year after pressure from the Japanese government. Softbank is expected to follow suit.

Travis Lundy, an analyst at investment research platform Smartkarma, said the main reason for investors to buy the stock is because it pays a 5% dividend, which is large by Japanese standards. If a price war hits earnings, that dividend could come under threat.

Listing the mobile business is a key part of Son's efforts to reposition SoftBank as a global tech investor. The move splits the company in two, allowing investors to choose between its mobile unit and its tech investment business, which includes a big stake in Alibaba.

Son, SoftBank's billionaire founder, has become a major player in the global tech industry after launching the almost $100 billion Vision Fund last year. But the fund's heavy ties to Saudi Arabia came under scrutiny in October following the murder of Saudi journalist Jamal Khashoggi.

The Saudi sovereign wealth fund is the biggest single investor in the Vision Fund.