West Texas Intermediate oil futures were plunging Monday afternoon—first into the single-digits and then into the red later in the afternoon. It was the first time since they started trading in 1983 that crude futures fell below $0. The previous settlement low was $10.42, which crude hit on March 31, 1986.

Oil has been tumbling for months, but fell off a cliff on Monday in part because of a technical shift. The May crude futures contract expires on Tuesday, and when futures expire their price tends to converge with the spot...