I don't think 'job-hoarding' is an appropriate term at all and approaches victim blaming.
These young people are each doing at least one job which used to be a career choice where a full time position would pay enough to sustain a person's costs of living.
They have been forced into multiple part-time roles by the relative change of living costs vs wages, by the casualisation of the work force into precarious part-time or zero-hours working, by the commoditisation of labour.