To the surprise and dismay of judges and prosecutors, Fairfax County's already busy courts are being swamped by an unprecedented - and largely unexplained - flood of new civil cases.

One Fairfax judge calls the more than 300 divorce cases heard each month recently in the county's circuit court "astonishing."

"In February, there were only 131 (divorce) cases," said Circuit Court Judge William Plummer. "But in every month following, the figure has been in excess of 300."

Other categories of lawsuits have tripled to 300 a month from last year, court officials said. Participants in civil suits who waited 60 days for a trial date two years ago now typically cool their heels for up five months.

"And the complexity of the litigation is mind-boggling," said Plummer. "A recent breach of contract case lasted 12 days. The American people apparently love to litigate."

Criminal cases are up, too, but to a lesser degree. According to Fairfax prosecutor Robert F. Horan Jr., the 261 indictments the county grand jury handed up in May was the largest number ever. The previous high was in March: 194.

"It's a significant increase," Horan said this week. "In all of 1978, there were 836 indictments. Just counting the March and May grand juries, we already have half of last year's total."

Law enforcement officials, watching to see if the trend holds up, offered a variety of explanations for the sudden bulge in their case load.

"The economy is generally down," said General District Court Judge F. Bruce Bach, who will be sworn in on Friday as a circuit judge to help out on the higher court. "We're experiencing expansion in all sorts of cases.

"People are generally more irritable. There will be as many as 200,000 traffic violations this year, as opposed to 97,000 in 1977, that may be largely attributable to gas lines and frustration."

Judge Plummer called population growth in Fairfax - the most rapidly growing jurisdiction in the metropolitan area - "the most obvious reason" for crowded courts.

"When you have higher density, you have a higher number of conflicts between people. But the great increases are in civil, not criminal class," he said.

Plummer said court dockets, the schedule of up-coming trials, are filling at "a tremendous rate in every kind of case you can think of" and said he expects the total rise in cases this year to be "several thousand."

"For the first time, we were forced to have two grand juries sitting simultaneously in May and well over 300 different charges were aired. And we didn't finish until 8 p.m.," the judge said.

"Times were when just one seating could have handled all the indictments before 5 p.m. There were 9,320 overall (circuit) court cases heard last year. There may be as many as 13,000 this year," he said.

Elsewhere in the Washington suburbs, only Montgomery County officials expressed similar dismay at longer trials and burgeoning caseloads.

"In the circuit court, there is a substantial backlog of cases and as much as a three-month wait for trial dates," said Administrative Judge David L. Cahoon of the Montgomery Circuit Court. "We have needed two additional judges to handle this for at least two years now."

Cahoon attributed much of the growth to requirements imposed by new legislation, particularly in divorce and custody cases.

Montgomery court officials will institute a computer system next week and have begun a case flow management system to cut the time spent on paperwork and to facilitate planning court schedules.

Spokesmen in Alexandria, Arlington and Prince George's County said their court caseloads had stablized with no significant increases in the number of indictments or in the length of dispositions and trals.

"I'm just happy it isn't happening here," said a spokesman for the Alexandria commonwealth's attorney's office.

In Fairfax, a new General District Court judge, Frank B. Perry III, will be sworn in July 1 to replace Bach when he moves to the circuit court bench. There will be nine circuit court judges and seven in general district court as a result of the move.

Prosecutor Horan noted this week that while there has been no significant increase in serious crime (murder, aggravated assault, rape), there has been an alarming growth in crimes of violence among juveniles.

"They used to be all robberies and minor property crimes," Horan said. "But we've noted a steady increase in the number of felonious assaults among juveniles."

Nevertheless, the prosecutor said, "We're still experiencing a rather typical suburban caseload pattern where rapid expansion and growing density exists."