Question

a pencil easer lose tiny pieces of rubber each time you use it why does it happen

Answer 1

because as rub the eraser, the rubber get wear and tear.

Answer 2

when you rub the stroke, there is friction b/w the eraser and your page. this friction as you would have studied, creates a wearing and tearing effect on either of those two things. You strecth your page, it doesn't have that effect on it more, but on eraser yes there is.
therefore the main principal behind this is FRICTION.