Financial markets once again have been seized by meme-stock mania. In mid-July, a frenzied rally erupted around a cohort of beaten-down companies, including Opendoor Technologies Inc., Kohl’s Corp.

An American Eagle ad campaign featuring Sydney Sweeney caught the attention of retail traders on Thursday, causing the stock to spike.

Trade deal hopes continued to run high after the US-Japan pact helped fuel more records for the S&P 500 and Nasdaq Composite ...