The Bernoulli'* equation is P+ρgh+ρv² =contant.
Taking ρ almost constant, in this case, h is also constant. Thus this equation becomes P +ρv² =constant = k (say)
→P =k-ρv²
This shows that if at a cross-section speed increases, the pressure decreases. When the car runs at high speed, the air just above the canvas is at a high speed but just below is static (with respect to the canvas). Thus the pressure above the canvas top is less in comparison to the inside. This results in bulging out of the canvas.
