It's been enough to catch the imagination of any John le Carre fan. Since Jan. 1, nine Western nations have expelled at least 60 accused Soviet spies. If one adds the recent expulsion of 18 Soviets from Iran, the world total of Soviet citizens expelled from foreign countries comes to 78. Most of the expulsions occurred within a matter of a few weeks, between early April and early May.
Spy novel buffs and others who try to follow events in the murky world of intelligence agencies raise a number of questions: Was the CIA behind the expulsions in the Western nations, coordinating and prodding? Was there an agent planted somewhere in the Soviet security establishment who put the finger on Soviet spies around the world - a ''deep throat'' who was telling all? Had the new Soviet leader Yuri Andropov, who only recently headed the Soviet secret service, the KGB, increased KGB activities to the point where the Soviets were simply running greater risks and getting caught at it?
The answer seems to be a combination of less sensational factors. They include a kind of chain reaction among Western nations and a growing awareness of persistent Soviet efforts to obtain Western military technology through espionage. It may not be satisfying to those who hunger for tidy conspiracy theories to explain complicated events, but this is the conclusion reached by a number of experts on the Soviet Union and some former intelligence officers.
An additional factor: In some nations, such as France, the expulsion of Soviet spies has made for good domestic politics.
Counter-intelligence officers in each of the nations involved may know of additional reasons for the expulsions. But in keeping with their tradition of minimal public disclosure, the officials of the Federal Bureau of Investigation and other such agencies are not saying much.
Most fascinating to some spy watchers has been the moderate reaction of the Soviets to all this. Not surprisingly, the Soviets have denied that they were engaged in ''impermissible activities'' in any of the nations concerned. They have blamed the expulsions most prominently on the United States. They say the US wants to prevent Moscow from developing good relations with the Western nations.
But it has been more than a month since France expelled 47 Soviet diplomats and other officials on espionage charges, and the Soviets have yet to retaliate against the French in any significant way. Indeed, Mr. Andropov indicated in an interview with the magazine ''Der Spiegel'' on April 25 that the Soviets had no intention of retaliating as they once might have done.
''To adopt reply measures with regard to the French working in the USSR would be the easiest thing to do,'' said Andropov. ''In exercising restraint, we are guided by the broad interests of Soviet-French relations . . . and by the interests of preserving detente in Europe.''
American officials say that the Soviets' highest priority at the moment is to win West European sympathy and to encourage a delay or disruption in the deployment of new US nuclear missiles in West Germany, scheduled to begin in December. Retaliation against the half-dozen West European nations that have expelled Soviet officials in recent months might do little to further the cause.
At the same time, US officials say, the Soviets place an extremely high priority on obtaining Western military technology through whatever means, legal or otherwise. Examples of top targets for Soviet spies are Western computers and microelectronic equipment that can be used in developing radar and nuclear missiles. According to American officials, the Soviets have managed, partially through legal purchases and partially through espionage, to narrow the gap between Soviet and Western technology in a number of fields.
In the case of France, Soviet intelligence officers appear to be most interested in advanced military and industrial technology. After the French threw out 47 Soviet diplomats and other officials, it seemed to create a chain reaction. Other nations, which feel that their small diplomatic missions in Moscow are highly vulnerable to retaliation, may have taken heart from the French example and expelled some of the most blatant Soviet agents. In some cases, it has involved matters other than industrial espionage. On April 29, the Swiss ordered the director of the Soviet news agency Novosti to leave Switzerland, charging that he had helped to train members of the nation's antinuclear movement.
In some cases, small nations like the Netherlands simply have quietly ordered a Soviet out in the hope that the lack of publicity might diminish the chances that the Soviets would feel obliged to retaliate.
For the US, which expelled three Soviets just a few weeks ago, throwing out large numbers of Soviets looks like an unlikely option. For one thing, the retaliation against American diplomats in Moscow could hurt the US out of proportion to the gains to be made. The Soviets would be certain to replace expelled spies, and the FBI would have to track possibly new methods used by new agents.
When it comes to Iran, some US officials say that the expulsion of Soviet diplomats can be attributed mostly to the Soviets' increased supply of weapons to Iraq.