George Soros and Stanley Druckenmiller’s investment firms trimmed their holdings in “Magnificent Seven” stocks before this year’s ebullient run-up in technology companies gave way to a major downturn in mid-July. Soros Fund Management sold some of its stake in Alphabet Inc. totaling $58 million, and about $15 million of Amazon.com Inc., according to regulatory filings for the three months ended in June. Druckenmiller was among investors paring stakes in Nvidia Corp., the chipmaker whose soaring...