Why should a rider avoid braking hard while leaned over in a turn?

In a turn, the tires are already using much of their available traction to hold the lean and corner. Braking hard at the same time asks the tires to do two demanding jobs at once, and they can exceed their grip and skid, causing a fall. The safe practice is to do most braking before the turn — slow down, then turn — and to straighten the bike as much as possible before braking firmly. If you must slow in a curve, brake gently and smoothly. The principle is that traction is shared among braking, cornering, and accelerating, so you cannot demand maximum of two at once.