The Bay Area’s once-scorching pandemic housing market continued to cool in June, with home prices plunging to the largest monthly drop for this time of the year in at least three decades. In June, the median price of existing single-family houses in the nine-county region declined 7% from the previous month — from just over $1.5 million to $1.4 million, according to data for the California Association of Realtors. That’s the steepest May-to-June dip ever recorded in the association’s regional home sales data, which dates back to 1990. What’s behind the record price drop? Real estate experts po…