Many delays are due to multiple reasons, the example above being typical.
1. Inbound aircraft scheduled for immediate departure again. That's what Low-Cost operators choose to do for their own cost efficiencies. So how can that be "outside the airline's control" ?
2. Mechanical fault, in this case with the AC, but it can be anything. Airlines are fully responsible to provide serviceable aircraft for the flight. So how can this be "outside the airline's control" ?
3. Crew scheduled tightly, so they now cannot complete the trip in hours remaining. No relief crew provided for such an eventuality. This is what Low-Cost operators choose to do. So how can this be "outside the airline's control" ?
4. Carrier chooses, in this case, not to cancel just the return sector, the only one where the crew would actually be out of hours, but the complete round trip, because it is more convenient to them to do so, and avoids costs of the aircraft standing overnight at an outstation. It also minimises any knock-on delays, all of which has to be reported to the relevant aviation authority in the periodic statistics. Better to report one cancellation than multiple delays. But all of this is an operational choice. So how can this be "outside the airline's control" ?