The current rash of musician-management disputes at symphonies is spreading. In the East, Philadelphia Orchestra musicians have overwhelmingly rejected management's latest contract offer and their attorney said a strike is probable. In Northern California, striking members of the San Jose Symphony recently caused the cancellation of the ensemble's first concert of the season. The 87 musicians, members of the American Federation of Musicians, Local 153, have been without a contract since Aug. 31, union spokesman David Schoenbrun said. The union has been negotiating with the San Jose Symphony Assn. and talks "are continuing," an association official said. Union officials said the musicians had been offered the same pay as last year for four fewer appearances this season.