As others have said, you're probably locking the rear wheel. This may be happening more than you're used to because of skinny tires, but more likely is that the brakes are more powerful on this bike and perhaps that your weight is distributed more forward, giving the rear wheel less traction. (btw, slick tires actually get better traction on pavement than knobby ones)

The best advice in general for braking is to get comfortable with using the front brake, and lay off the rear. It's far more effective at stopping the bike (Sheldon used to claim that it gives half the stopping distance of the rear wheel).