It is when an employer’s behaviour is such that it allows the employee to assume dismissal. For example, suddenly changing working hours from 9.00 -17.00 to17.00 – 01.00, or perhaps a radical change in job specification being imposed.
Whether or not the employer’s behaviour is serious enough to result in constructive is a matter for an employment to decide, but if constructive dismissal is found then it is wrongful dismissal.