[cstreit]

This could be pad knockback.

If a wheel bearing is loose or the hydraulics are losing a little residual pressure then that first pump moves the pad back into contact with the rotor. THe second applies the pressure.

If this mainly happens after a longer delay using the brakes or after an especially rough section of track then almost certainly is.

This can be solved by fixing the loose wheel bearing... ...adding anti-knockback springs or seals - or a residual pressure valve that holds a few psi of pressure in the system.