Maybe their history forms part of their distrust of intruders.
//In January 1880 an armed British expedition to the island, led by 20-year-old Maurice Vidal Portman, the local colonial administrator, arrived to conduct a survey of the island, and to take a prisoner, in accordance with British policy regarding unwelcoming tribes at the time, which was to kidnap a member of the tribe, treat them well and give them gifts, and release them back to the tribe, hoping to demonstrate friendliness. In 1879 Portman's expedition of the island is believed to be the first by outsiders. While the Sentinelese tended to disappear into the jungle whenever outsiders were spotted approaching, Portman's expedition found an elderly couple and four children after several days. They were taken prisoner and brought to Port Blair by force to try and befriend them. The adults died, possibly from contracting diseases to which they did not have immunity. The four children were given gifts, and released on the island where they disappeared into the jungle. The British did not subsequently try to contact the Sentinelese and instead focused on other tribes.//
I'm glad the fishermen who took this well meaning meddler to his death have been arrested.