Casualisation of a workforce is the reduction in full or part time employees and their replacement with employees who are called in on an as-needed or casual basis.
This can reduce the employees working conditions by reducing the commitment from the employer to them, and giving the employer opportunities to control them by reducing their hours.
Casual workers can be more difficult for employers to manage as they have no guarantee of finding available employees at any time, but they have the advantage of only employing people when they have the work for them.