Cars are usually set up with front brake bias. This is because during braking, there is a load transfer to the front tires meaning the front brakes are the bigger component of your stopping power. Also, as the load is transferred off the rear, they are more likely to lock, so you actually want less braking back there. The tractive force of the tires is a function of the normal force (or weight that is pushing it down) on each tire. The more normal force, the more tractive force available to an extent.
Tires have what is known in classic vehicle dynamics as a friction circle, thought data shows it's never truly a circle, which they have to operate in as shown below in the generic example which I took from driftingstreet.com:
What the circle demonstrates is that if you ask a tire to do either pure acceleration or pure braking by either spinning the tire or locking the tire, there is no traction left for cornering. This is what Dubgurl was referring to. Lock the rear first, it's easy to spin-out. Lock the front, you lose steering.
However, as Larry pointed out, we can influence the weight transfer on our bikes by shifting our body weight. Without shifting our weight, the rear brake could easily lock as the load is transferred to the front due to the reduction in normal force. By shifting weight, we can more heavily brake the rear without locking it while also applying heavy front brakes.