Why some kidnapped Westerners were set free. LEBANON'S HOSTAGE-HOLDERS

The recent abductions and subsequent release of three Western aid officials in southern Lebanon seem to have taken place outside the mainstream of politically-motivated hostage-taking in Lebanon. An outcry from the many Lebanese and Palestinians who benefit from international aid led to pressure that helped free the Westerners.

Their release, however, would not appear to have hopeful implications for other Western hostages in Lebanon. Almost all the 18 Westerners still believed alive in captivity, are assumed to be in the hands of clandestine pro-Iranian Shiite Muslim groups linked to Hizbullah (Party of God). The groups have demanded the release of all Lebanese and Palestinian prisoners in Israeli jails and the freeing of convicted Shiite extremists in Kuwait. The most recent victim of such politically motivated kidnappings is US Marine Lt. Col. William Higgins, taken hostage Feb. 17.

The abduction of two Scandinavian relief workers near Sidon on Feb. 2, and of British aid official Peter Coleridge in the same area last Thursday, however, appeared to be in a different category from the outset.

The captors in both cases were Palestinians, who are not believed to have played a role in the major political kidnappings. Both incidents appeared to stem from local conditions in the Sidon area - conditions quite different from those in west Beirut, where virtually all the current hostages were seized. Sidon is largely controlled by a Sunni Muslim militia that has good relations with Palestinian guerrillas who have reestablished a powerful presence in and around the refugee camps on the city's outskirts.

The two Scandinavian officials of UNRWA, the UN agency that serves Palestinian refugees, are believed to have been abducted by Palestinians with a personal grudge against the agency. A power-struggle among Palestinian factions, sources say, held up their release until March 1.

Peter Coleridge, Mideast coordinator for the British charity Oxfam, was detained by armed Palestinians, reportedly from the radical Abu Nidal group, in the Ain Hilweh camp last Thursday. He and a Syrian colleague, Omar Trabulsi, were freed Tuesday.

In an interview after his release, Mr. Coleridge said his captors stressed he was not a political hostage, but was being questioned for security reasons. He had aroused suspicions by taking pictures in the camp. ``It's not as though I was just taken off the streets,'' he said. ``They had a legitimate concern'' to establish whether ``I was a spy.''

One exception to the politically motivated kidnappings is that of West German Ralph Schray. His abduction Jan. 27 was widely seen as linked to West Germany's trial of Shiite radical Muhammad Ali Hamadei in connection with the 1985 TWA hijack.

But Mr. Schray was released last week - apparently through Syrian pressure. His kidnapping in west Beirut, patrolled by Syrian troops since February 1987, was seen by Syria as an intolerable challenge. But Syria's writ does not hold in the far south where Colonel Higgins was seized.