what you described is not constructive termination nor is the OP's termination a case of constructive termination.
constructive termination is where an employer unilaterally changes the conditions of employment (hours, pay, etc), conditions of the workplace, and/or tasks that the employee is required to perform with the intent to force that employee to quit. this is done by an employer that doesn't want to pay a severance or the employer cannot find just cause.
examples of constructive termination is cutting the hours of a 25-year full-timer to 4 hours a day despite his/her objection or demoting the centre manager with 20 years experience into a part time preload trainer, done with the intent of forcing these employees to quit.
what you described is hardly illegal provided it's for just cause (insubordination, dishonesty, misconduct), unless the employer systematically harasses the employee (like entrapment).