Unnerving braking experiences; sudden braking increase.

I experienced something a bit unnerving yesterday on a
bike I have been riding a couple years. The bike is a
nifty steel sport frame. The brakes are Ultegra 49 mm
reach with Kool Stop pads. On descents I was waiting a
bit longer before braking at turns, using more braking
force, but still using progressive force, rather than
slamming them. At a later point in the brake
application, the braking force suddenly increased
sharply. This happened more than once. Had not seen
this before, and need more experiments. Any comments on
this sketchy account?

--
Michael Press