NOT FOR THE first time, Virginia Sen. Paul Trible Jr. has managed to leave Republican colleagues as well as Democratic foes wondering exactly what he is doing. After a career in Congress including three terms in the House and one in the Senate, Mr. Trible says "it's time to look to the future" -- and time to vacate his Senate seat. The reasons he gave were1) family, which he wants to see more of, and2) legislation frustration, which he's seen too much of. To this you could add a third consideration, one quite visible to Virginia's junior senator. Charles Robb, the popular former Democratic governor, has been sounding more and more interested in the 1988 Senate race. He is considered a strong favorite to topple Mr. Trible if the two were to go head to head.

Virginia Republicans are still picking up the pieces from two back-to-back Democratic sweeps of the top state offices. They are now wondering whether Mr. Trible's obvious if not yet publicly stated interest in the governorship might be their salvation in 1989. Edward S. DeBolt, an Alexandria pollster and media manager of Sen. Trible's last election, believes the senator's eye is on Richmond now: "The idea that Paul would disappear and just become a corporate lawyer and make a lot of money is not a likely scenario. The guy's a scrapper. He's a steely, intense politician."

As it happens, there are a number of former officeholders in Virginia who are marking time practicing law but who would rather be scrambling to be in the Senate or the governor's office. Sen. Trible's announcement creates for them, and for others who are eyeing the political scene, a whole vista of new choices. Virginia politics were thoroughly predictable in the Byrd Old Days. Surprises are the order of the day now.