Why is a starting current in a DC motor very high?

A rotating d.c. motor generates a back-emf which opposes the supply voltage and reduces the current drawn by the motor. When the motor is stationary, it cannot generate this back emf and, so, the only opposition to current is the resistance of its windings which is relatively low. So, on startup, the current is large; as the machine starts to run, the resulting back emf, acts to reduce the current.