On Halloween 10 years ago today, EgyptAir Flight 990 nose-dived into the ocean off Nantucket from 33,000 feet, leaving the families of 217 children, grandparents and lovers forever haunted by the question: Why? “You just wish you knew what the reason was. Some things break your heart so much,” said Bobbie Bergman of Colorado. Bergman’s sister Paula Barnes, a one-time Marblehead resident, perished with their parents William and Tamsin Barnes and several family friends about 1:52 a.m. on Oct. 31, 1999, a half hour into what was to be a 10-hour passage from New York to Cairo.