Just how hard are you applying the rear brake? If it feels like your
rear wheel is going straight while the rest of the bike is going through
a turn, you're skidding the rear wheel. And yes, there is a "physics"
reason for this. It's called inertia. An object in motion tends to move
in a straight line unless acted upon by another force. (sic.)

If your front wheel slides out, it too will go in a straight line. With
disastrous results. Actually, the best thing is not to brake at all in a
turn, Do all your braking before.

The tires have a limited amount of "grip" on the pavement, any that's
diverted towards slowing the bike down is being taken away from the
amount holding it in a curved path.

If you must apply brakes in a turn, do so very gently, and it's best, in
this case, to favor the rear. If that gives away, you have a much better
chance of riding it out.

May you have the wind at your back.
And a really low gear for the hills!
Chris

Chris'Z Corner
"The Website for the Common Bicyclist":
http://www.geocities.com/czcorner