Without knowing any background, miles on the bike, miles on the tires, did you hit a curb, etc... it's hard to say.
I've had the same problem several times, and it's always been tire wear or air pressure. I have an America, and I've been running Avons for the last 18000 miles or so and although they stick like glue, they also "cup" very easily. A panic stop skid will put a flat spot on your tire that you may not notice but will affect the ride as the tire continues to wear and scrub unevenly. Rear or front.
Put the bike on a lift and run your hand over the tread. You can feel the cupping even though the tire looks good...
