Got the magna serviced yesterday, and the mechanic found that the power steering belt had self destructed.
The problem being a new one was installed 10k km ago, when the timing belt was changed....

Warranty covered the bill (part warranty)...but my question is....how rare is this?
Is it because the belt was old stock, incorrectly installed....?

I know there are many reasons for this, im just crowd asking what the most popular answer is..

Got the magna serviced yesterday, and the mechanic found that the power steering belt had self destructed.
The problem being a new one was installed 10k km ago, when the timing belt was changed....

Warranty covered the bill (part warranty)...but my question is....how rare is this?
Is it because the belt was old stock, incorrectly installed....?

I know there are many reasons for this, im just crowd asking what the most popular answer is..

Incorrect installation will destroy a belt quickly. It needs to sit in each pulley correctly, misalign the ribs on one pulley and it gets shredded.
But you really need to see the belt to be able to tell. If it is shredded along one of the ribs, misalignment is the cause.
These belts are usually good for 100 thousand Km +.

Yes, the edges looked like a Labrador had been playing with it for a week....
Leaves me wondering how long the timing belt (installed by same person) will last me ....

I'll go with incorrect installation. I've mistakenly installed a polyrib mistaking the outer edge as a groove. Ran for a while but eventually cut through and shredded the belt. Unless the belt was overtensioned t o the point of snapping the cords in it but I would say very rare . I've got 100 000 on mine now and I'm about to replace both and the idlers

Yes, the edges looked like a Labrador had been playing with it for a week....
Leaves me wondering how long the timing belt (installed by same person) will last me ....

Timing belt is a completely different style. It doesn't fit into grooves or a Vee pulley like the belts on the outside of the engine do. It would be hard to 'fit' it incorrectly but getting the timing and tension right does require considerable care. (Something the mechanic has not displayed so far). Although, if you've already got 10,000 ks out of it without problems then its probably OK. A dodgy timing belt installation would probably show up pretty quickly.