T-Mobile US Inc. (TMUS) is not used to double-digit moves. Now the stock is having its worst day in 13 years.

Shares fell 13% to $149 at midday Friday. That puts the stock on track for its steepest one-day drop since it began trading under the T-Mobile name in May 2013.

The selling followed a deal by Space Exploration Technologies Corp. (SPCX). SpaceX agreed Thursday to buy nationwide 800 megahertz spectrum licenses from Grain Management.

These low-band airwaves travel long distances and pass through walls. SpaceX plans to pair them with its Starlink satellites. As a result, wireless carriers could face a new competitor.

The deal still needs Federal Communications Commission approval.

Peers AT&T Inc. (T) and Verizon Inc. (VZ) fell 11% and 10%, respectively.