By Selina Wang, CNN Business

(CNN) -- SoftBank's shares plunged more than 7% Monday, wiping out roughly $8 billion in value, as investors reacted to reports that the Japanese conglomerate has been making massive bets on major tech stocks.

The company's outsized role in Silicon Valley has been well documented for years as it shifted its focus away from telecommunications and toward strategic investments in flashy technology startups — including Uber and WeWork — through its $100 billion Vision Fund.

But weekend reports in the Financial Times and the Wall Street Journal suggest that founder Masayoshi Son has turned his attention to Wall Street bets on America's heaviest hitters, after raising billions from offloading assets in recent months.

The company bought roughly $4 billion worth of options tied to underlying shares it had earlier purchased in tech firms like Amazon, Microsoft and Netflix, according to the Wall Street Journal. Citing anonymous sources, the newspaper said that the options generated an exposure of about $50 billion.

(The options apparently in play here are call options, or contracts that give the investor the right to buy a stock at an agreed price; they become profitable if the share price rises above that level.)

Both the Journal and the FT — which first reported the story — have suggested that SoftBank's options positions partially explain the recent tech rally on Wall Street. Prior to sell-offs on Thursday and Friday, the tech-heavy Nasdaq Composite was frequently reaching record highs. The S&P 500, where a handful of tech stocks account for a quarter of the benchmark's weighting, was climbing to records, too.

The FT, which also cited anonymous sources, reported that SoftBank is sitting on trading gains of about $4 billion from the bets. But such trading is also risky, since they could lead to big losses if equity markets keep falling. The FT report cited a banker who described the company's strategy as a "dangerous" bet.

SoftBank's losses on Monday accelerated into the afternoon in Tokyo, with shares having their worst performance since the pandemic shocked global markets in March.

The company has recently indicated an interest in investing via the stock market: SoftBank announced in August that it would start a new unit for public investments with $555 million in capital (555 is slang in Japanese gaming culture for "go, go, go".) Son owns 33% of the asset management firm.

The purchases are also notable given how much cash SoftBank has been raising this year. Last month, the company announced that it would sell more than one million shares in its Japanese mobile carrier affiliate SoftBank Corp., worth 1.47 trillion yen (nearly $14 billion). That asset sale came on top of plans the firm announced in March to raise some 4.5 trillion yen ($42 billion) by selling assets.

SoftBank said last month that it believed it was "necessary to expand cash reserves ... to ensure flexible options to respond to changes in the market environment."

Prior to Monday's fall, SoftBank's stock was up about a third this year. It was a remarkable turnaround for the company, which struggled in March and April as the pandemic and the ensuing restrictions on travel and social interaction took a toll on many of the startups it has invested in.

In May, SoftBank reported an annual operating loss of 1.36 trillion yen ($12.7 billion) — its worst ever. The hit was driven almost entirely by the Vision Fund.