I'll go with tiotally random.
Depends on a lot of things.
As a sort of rule of thumb, I usually buy computer psu's in batches of a dozen or so. There are the component I replace most frequently.
So it wouldn't surprise me if the psu on the printer died. On the other hand the psu in my 3d printer gets a lot less use than the psu in my computers, so by rights it should last longer too :-)
I'd think 3 years isn't unreasonable.