The yuan’s growing influence in Hong Kong’s economy is bringing a sea change to the city’s interbank market, with one outcome being the recent accelerated weakening of the Hong Kong dollar that is likely to complicate efforts to defend the currency peg. A recovery in yuan this year saw heavy investments in stocks that make up the Hang Seng Index and Hang Seng China Enterprises Index, which increased liquidity in the economy and anchored the city’s interbank rates at...