As the current through the field winding decreases the field strength also decreases and the back-emf is reduced, as a result the motor speeds up.
When the motor begins to speed-up, an additional back EMF is generated due to induction which reduces armature current. This also results in reduced strength of the field flux. The back emf generated is not enough to reduce the armature current which could stop the acceleration of the motor and the motor further accelerates.
