There are two things to think about here.
(1) On a bike, engine braking only acts on the back wheel. If you press the clutch while you are braking hard, you change the "balance" of the braking force between the front and back wheels, which might cause problems especially if you are turning as well as braking.
(2) if you start to lose control of the ...
My previous answer was mistaken (I opted to leave the mistake visible, rather than edit it).
declutch intransitive verb de·clutch | \ (ˈ)dē+ \
: to disengage a clutch
: to put out of action by releasing a clutch
"releasing a clutch" is the place I got confused. Although we release a lever on the handlebars, the releasing of ...