The United Auto Workers union has widened its strike against General Motors, the lone holdout among the three Detroit automakers, after reaching a tentative contract agreement with Jeep maker Stellantis.

The escalated walkout began Saturday evening at a Spring Hill, Tennessee plant, GM’s largest in North America, just hours after the Stellantic deal was reached. Its nearly 4,000 workers join about 14,000 already striking at GM factories in Texas, Michigan and Missouri.

The UAW did not immediately explain what prompted the new action after 44 days of targeted strikes. The added pressure on GM is substantial as Spring Hill makes engines for vehicles assembled in a total of nine plants as far afield as Mexico, including Silverado and Sierra pickups. One plant already on strike it supplies with engines, in Arlington Texas, makes full-size SUVs including the Tahoe and Suburban. Vehicles assembled at Spring Hill include the electric Cadillac Lyriq, GMC Acadia and Cadillac crossover SUVs.