The unexpected and thus far unexplained ouster of Soviet President Nikolai Podgorny from the Politburo this week, an event of considerable interest in the international scene, dramatizes once again a truly daunting fact about the Kremlin: outsiders have not a clue as to what is really going on inside.

Rumors about changes in the Soviet leadership come and go in Moscow like the seasons, traded by foreigners and the closed circle of Russians they talk to, and there always seem to be theories circulating in the West. So it was that Podgorny's supposedly waning influence and other infirmities - he is 74 - were cited, at times in the past, as evidence of his imminent demise.

But not lately.

Indeed, Podgorny had rarely been as prominent as he was barely two months ago when he traveled through southern Africa, on a tour that was widely regarded here and abroad as a triumph for Kremlin diplomacy in one of the world's most sensitive regions.

By all accounts, Podgorny acquitted himself well there and got what seemed a routinely warm welcome home. So dealing with why Podgorny was so unceremoniously sacked now - if in fact ,as everyone here believes, he did not go willingly - can only be a matter of speculation, informed perhaps but still only conjecture.

The significance of his departure may become clear when his successor as the country's head of state is revealed. (Podgorny has not yet been formally dropped from the post, although his name all official pronouncements.) That should tell the effect of the change on the person who matters most in determining Soviet policy: Communist Party General Secretary Leonid Brezhnev.

But unless Podgorny himself or some other Kremlin principal writes his memoirs, as Nikita Khrushchev did after he was dumped in 1964, the odds are that those outside the Kremlin will never actually know what the conflict was that brought him down. And without knowing such things it is impossible to say when they might happen again - a serious business for President Carter and other Western statesmen who need to chart Kremlin actions in order to pursue their own.

There are a number of possible explanations of why Podgorny had to go at this moment and why - as a stalwart of so many years' standing - he was not allowed to go more gracefully. The ideas come from Soviet journalists and tipsters, diplomats of various countries and foreign correspondents. Here they are in ranking order of likelihood:

Brezhnev has decided he wants the job of president, to give him as much stature as the six of his Eastern European colleagues who now hold both the party's top title and the senior state post. The issue came to a head because the change was to coincide with the announcementof a new Soviet constitution. Podgorny balked, and in a showdown Brezhnev won.

Brezhnev is consolidating his already vast powers and wanted to move Premier Alexei Kosygin, who is 73 and suffered a serious heart attack last summer, into the essentially ceremonial post of the president so an all-out Brezhnev man could be made prime minister. Once again. Podgorny resisted to the point of losing out altogether.

Brezhnev has another candidate for the job in his faithful No. 2, Andrei Kirilenko, or the party's longtime senior ideologist, Mikhail Suslov. Podgorny was never especially close to the party leader, so at the first available opportunity he was fired. He could have stayed on in a lesser capacity but refused.

One of the relatively younger men in the Politburo - there are three still in their 50s - will be promoted into a position that affords greater visibility if only vague authority. As with the other theories, it was Podgorny's refusal to step down that cost him some kind of a consolation prize.

A full-fledged Kremlin power struggle is under way in which the ouster of Podgorny is only the first round of what wcould be many other big changes. Exactly who will emerge as president has not yet been decided.

This last version is perhaps the most exciting to those Kremlin-watchers for whom the recent years have been frustratingly stable. The most important issue underlying all these scenarios, however, is the succession to Brezhnev himself. The end of Podgorny must be seen as part of that process, no matter what the reason for it.

Brezhnev is plainly preparing a legacy. After more than a decade of internal debate, he has finally managed to bring forth a new constitution to replace the one Joseph Stalin had written in 1936 and a national anthem to replace one in which the words were as discredited as Stalin was.

The 70-year-old party leader obviously has a vision of how the world and his Kremlin entourage should look as he, at the height of his power but in uncertain health, faces the future.

The one thing known now that could only be guessed at before is that Nikolai Podgorny did not fit into the vision.