THE release of Anglican envoy Terry Waite and United States hostage Thomas Sutherland on Nov. 18 caught many diplomats and observers in Beirut by surprise.Until the Nov. 17 edition of the Tehran Times heralded the latest releases, there had been fears that the process begun in August, when British hostage John McCarthy gained his freedom, might have ground to a halt. "I certainly found it curious," admits British ambassador in Beirut David Tatham. "Why should they release hostages now, while south Lebanon is burning?" Recent weeks have seen Iranian-backed Shiite radicals in south Lebanon engaged in an escalating series of hostilities with Israel. Both sides are thought to be parties in the complex, low-profile hostage negotiations, conducted by United Nations envoy Giandomenico Picco, which have seen five more Western hostages freed since Mr. McCarthy. Perhaps because of the hostilities in Lebanon, there has been little progress recently on further exchanges of captives between the Israelis and the Lebanese Shiites. Israel is still waiting for evidence on the fate of four of its seven servicemen missing in Lebanon before releasing another batch of Shiites held in an Israeli-controlled prison in South Lebanon. Iran, universally acknowledged to have close links and influence with the Lebanese kidnappers, is also a key party in the delicate negotiations. But Tehran has fiercely opposed the Middle East peace process that got under way in Madrid on Oct. 30 - another reason why diplomats feared hostage releases might have been put on hold. So the sudden release of two such important hostages as Mr. Waite and Mr. Sutherland - and the news they carried of other apparently impending releases - came as startling evidence that the hostage release process was still moving ahead, and could be completed more swiftly than even optimists dared hope. There is rising speculation among both Arab and Israeli hostage-watchers that the Beirut kidnappers may have discarded the Israeli side of the equation and begun moving ahead to end the hostage affair unilaterally. The Israelis have released none of their Lebanese prisoners since before the last US hostage, Jesse Turner, was freed Oct. 22. Israeli Prime Minister Yitzhak Shamir made it clear no more Shiites would be freed until the Israelis get further proof about the fate of their servicemen. If what the kidnappers themselves told Waite and Sutherland about the imminent release of the three remaining US hostages turns out to be true, it would imply the kidnappers do not intend to wait for further releases of Lebanese held by the Israelis. Why the kidnappers may be abandoning the triangle - if indeed they have - is far from clear. The general assumption so far has been that the captors' main motive in joining the hostage release process has been to secure freedom for the inmates of the al-Khiam prison in south Lebanon. But some analysts believe that may have been more of a face-saving device than a driving motivation, since the hostages were not seized with that purpose primarily in mind. Since they probably hold one living Israeli serviceman and have the remains of several others, the Shiite radicals know they can strike a separate deal with the Israelis any time. All observers agree that the main impulse for the kidnappers to free their hostages must come from their Iranian mentors. For many months, Lebanese Shiite and Iranian sources in Beirut have said Tehran is determined to close the hostage file. Iran's own motivation is widely held to be President Hashemi Rafsanjani's desire to move toward more normal relations with the West. Now that Waite - the last British hostage - is freed, diplomatic sources expect London to upgrade relations with Tehran to the ambassadorial level soon. All the signs since August have been that Iran, Syria, and others with a say in hostage releases have decided that nothing should be allowed to disrupt a process which started to get under way last year, with the freeing of Americans Robert Polhill and Frank Reed, but was then derailed by the Gulf crisis. The same determination to leave obstacles to one side has been evident in the fact that six hostage releases have gone ahead since August despite the problem of the Hamadei brothers, Muhammad Ali and Abbas Ali, jailed in Germany on hijacking, murder, and kidnapping charges. Hamadei clansmen, strongly linked to the Iranian-backed Hizbullah, are believed to be holding two German aid workers abducted in 1989, trying to use them for an exchange deal which Bonn rejects.