High likelihood of either tripping off the supply circuit on overcurrent or causing major equipment damage due to overtorquing.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

Because a synchronous motor doesn't slip which means it can only work as a synchronous motor at full speed.

It is a similar process to synchronising a generator set to the grid, and for much the same reasons.

With a generator you want to match the grid frequency and voltage, then close the generator breaker.

With a synchronous motor, you close the motor breaker and the motor is accelerated to speed via a squirrel cage winding built into the rotor pole faces (other starting systems are possible). The rotor field breaker is then closed to apply the field when the rotor is at maximum speed (could be 90% as you say). The motor then accelerates to synchronous speed and full load can be applied. Larger motors have to be started unloaded, as the squirrel cage often has relatively poor torque.

If you apply the field at too low a speed the motor will try and draw a high current from the supply and will trip on over current.

As soon as the DC is applied to the field on rotor, the rotor is pulled in to synchronism with the power supply. This causes a jerk on the rotor shaft that amounts to torsional stress which can reduce the life of the shaft.
Higher the deviation from the synchronous speed, higher the stress.
This is the reason why one would like to interlock Field DC application with motor speed (not less than 95% rated).