The only time I've fishtailed when not braking was in high cross winds when my headset was loose and I was going too fast. If there wasn't wind or something slick on the pavement, check your bike.

My scariest fishtail - I braked too hard on a downhill (Mt Diablo) on wet pavement when a car coming uphill rounded a corner in my lane. I was flashing on Beloki's horrible fishtailing and crash at TdF since that was how I was fishtailing - and flashed too on smashing into the grill of the still approaching car - before I just managed to get the bike under control and steer around the car.