At no load the field current (which is also the armature current) is very small and hence the useful air-gap field flux. Also the torque is very small.
As N ∝ 1/Ф the speed rises to excessively high values and it is mechanically very harmful for machine.
At high speeds due to centrifugal forces on the rotating parts they may get damaged. Hence DC series motor should never be started on no-load.