Stocks, bonds, gold and bitcoin—assets that rarely move in unison—have all been surging this spring, an everything rally that leaves investors confounded about how to play the plodding U.S. expansion and vulnerable to sharp reversals in fortune.

Major U.S. stock indexes have soared to records this month, reflecting some investors’ confidence in the continued U.S. economic recovery along with expectations that large technology firms will accrue further market-share gains. At the same time prices of bonds, which often decline...