Road Hog
There are any number of reasons why this has happened from human error to mechanical failure of sort. It is 3-6 months too soon for anyone to speculate why.
Why are you thanking Monarch in a sarcastic manner? An aircraft operating for them has encountered an incident, cause unknown, no-one hurt, 2 hour suspension to operations, things now back to normal with some delays as a result - how is that the fault of Monarch??
One pax is quoted on the BBC as stating "it's the oldest aircraft I have ever flown on" - as if aircraft age/state/condition is the cause - what has that got to do with an incident the cause of which is unknown. The flight had taken place and landed - age of the aircraft is irrelevant.