Wheels don't typically go out of whack like that for no reason. Whenever that has happened to me, I've either hit something, crashed, or broken a spoke. Without knowing any of the specifics of your wheel, the only other thing I can think of is that the spokes may have been over-tensioned. That could result in an unstable wheel that has a propensity to buckle. A properly built wheel is remarkably stable, and will not go out of true when you're "just riding along."
You say the shop trued the wheel but didn't find anything else wrong with it. I would have them verify that they properly tensioned the spokes when they trued your wheel. If the spokes are fine, then I wouldn't worry about it.
By the way, you might try posting this in the Bicycle Mechanics forum. There are folks there that know a hell of lot about wheels. I'm just a newbie wheel builder.