The explosion at the Milwaukee Brewers' training camp was triggered when a pilot light from a heater ignited a pocket of natural gas released as the heater was being installed, says a Chandler, Ariz., city official.

Nine persons were injured in the clubhouse blast Thursday. Four, including three Brewers coaches, remained hospitalized yesterday for treatment of burns. Tony Muser and Herman Starrette remained in serious but stable condition; Larry Haney was upgraded from serious to fair.

The Chandler Fire Department's investigation determined that plumbing firm employe Jeff Sutton was installing a heating unit in the coaches' room of the clubhouse when he bled the natural gas lines to remove air and allow the heating units to operate, said John Pinch, assistant city manager. Gas accumulated near the ceiling but was drawn down by the unit's fans, he said.

"The explosion and flash fire occurred as the gas and air mixture reached the burner's flames," he said.

Sutton, 22, was listed in fair and stable condition . . .

The New York Yankees have received an offer to move to New Jersey that would guarantee the team 2.2 million in attendance and pay off its lease on Yankee Stadium ($1.75 million a year through 1999), the New York Daily News reported. The offer was made by officials of the Meadowlands, home of Giants Stadium. New Jersey voters last year approved construction of a major league baseball stadium.

Vincent Tese, chairman of New York State's Urban Development, told the Daily News that Yankees owner George Steinbrenner is considering the move. Steinbrenner told the Daily News the team is negotiating with New York City regarding unkept promises to improve the Yankee Stadium parking situation.