Rare. Happened to me twice in 40 years riding, and the second one, at 42mph downhill with a following car, scared me so much that 10 years later I still instinctively touch the brakes when I get over about 35mph.
The first time was probably due to badly packed front paniers on a tourer, the second - no idea, I was on a light road bike with no obvious cause.
As others have said, there are tricks involving lifting weight off the saddle and knee-clamping the frame, but you learn about these afterwards and I've thankfully not had any cause yet to put them into practice.