The standard metaphor for Soviet diplomacy for many years was Nikita Khrushchev's remark that his foreign minister, Andrei Gromyko, would sit on a block of ice until it melted if ordered to do so.

Since such drip-by-drip diplomacy was Moscow's chosen method most of the time, the Kremlin's dramatic reversals of course in the past week on a possible superpower summit in Washington are a surprising departure from the norm. How this happened, and why, are subjects of intense interest and much speculation among U.S. policymakers and other observers of the Soviet Union.

Interviews with policymakers who participated in Secretary of State George P. Shultz's talks with Soviet officials in Moscow last week suggest a growing consensus that the turn of events was the product of Soviet leader Mikhail Gorbachev's highly personal negotiating style, plus a critical misjudgment of the Reagan administration by Gorbachev and some of his key aides.

The swiftness of the Soviet turnabout suggests to these analysts that Gorbachev is very much in control, since he evidently did not have to spend time persuading other leaders to change the Soviet posture once it became apparent they had not succeeded in winning concessions from the Americans on the Strategic Defense Initiative (SDI) and had embarrassed themselves from a public relations standpoint.

At the same time, a series of other flip-flops and odd soundings from the Soviet Union has left officials wondering if a new phase of competition for power is under way within the ruling Politburo or whether there has been an outbreak of bureaucratic politics between the Communist Party secretariat, the Foreign Ministry and the Defense Ministry, much like here.

"I think they overstepped in trying to {get} a U.S. move on SDI as a precondition for a summit meeting, and when they saw the reaction, they turned around," said a U.S. official who participated in last week's Moscow meetings.

To recap what happened:

During Foreign Minister Eduard Shevardnadze's trip here in mid-September, the United States and the Soviet Union agreed to hold a summit meeting of their leaders this fall, during dates to be established in Shultz's Oct. 22-23 mission to Moscow.

Shultz said that Shevardnadze, who arrived in Washington last month with a Gorbachev letter conferring on him full negotiating authority, proposed and drafted this particular agreement.

But last Friday in the Kremlin, Gorbachev unexpectedly did not set the summit date. Instead, he spoke of the need for agreement at the next summit on "key provisions" of future pacts on strategic offensive arms and space arms. When Shultz replied he could give no assurance on the latter point, which would put limits on Reagan's SDI, Gorbachev said he did not "feel comfortable" setting a date.

As the meeting proceeded, a U.S. official sitting across the table noticed that Shevardnadze, sitting on Gorbachev's right, looked particularly unhappy. The veteran former Soviet ambassador to the United States, Anatoliy Dobrynin, sitting on Gorbachev's left as chief Kremlin adviser on international affairs, displayed no such concern. Some U.S. officials theorize that several of Gorbachev's proposals last Friday were worked out by Dobrynin with another participant in the meeting, Marshal Sergei Akhromeyev, chief of staff of the Soviet armed forces.

This Tuesday, only four days later, Soviet Foreign Ministry spokesman Boris Pyadyshev announced the surprising news that Gorbachev would come to Washington "in the near future." At the same time, Shevardnadze called in U.S. Ambassador Jack F. Matlock to say he was ready to fly to Washington Friday to continue the discussion of arms control and the summit. Yesterday Pyadyshev announced that a summit meeting has been assured.

All this took place, according to U.S. sources, without any substantive negotiations in the week since Shultz left Moscow.

Another indication that strange things are going on within the Soviet hierarchy was illustrated for U.S. officials by a surprising set of flip-flops regarding the Soviet position on space and defense. In that case, a position set forth by Shevardnadze on space research in Washington last month was suddenly repudiated "on orders from Moscow" at the Geneva talks in mid-October by chief Soviet space and defense negotiator Yuli Kuznetsov. When the Shultz party arrived in Moscow last week, however, Gorbachev, Shevardnadze and others reverted to the mid-September position, and one senior Soviet arms negotiator even denied to a U.S. counterpart that the Kuznetsov demarche had ever taken place.

U.S. officials can only speculate about what this disarray reflects. Some believe that with the free-wheeling style of Gorbachev, the Kremlin inadvertently has caught "the American disease" of divided counsels and bureacratic combat. Whatever the case, it is clear that the familiar glacial pattern of Soviet diplomacy has, at least for now, been replaced by something far more accommodating, but also far more difficult to fathom.

Staff writer R. Jeffrey Smith contributed to this report.