They usually blow for the correct reason - that the transformers gets too hot - the issue is why it got too hot.
Sometimes it's due to a faulty transformer (shorted turns as I mentioned above), or it could be something external, or as simple as the transformer was operated outside it's specifications. Often you don't know what the reason might have been, and it 'might' just have been coincidence?, but I suspect there's mostly a reason somewhere - such as poor ventilation?.
I had a big problem with a Sony amplifier, still under warranty - it was a really nice big 100W+100W RMS amplifier, and belonged to a young guy who obviously liked his music
LOUD
It came in for repair, and the first thing I noticed was how light it was - it was a big chunky amplifier, and I expected some heft to it.
When I took the top off, the reason was obvious, a tiny little toroidal transformer sat on a special metal mounting bracket with lot's of empty space around it. The bracket was there to make it fit in the large space. Needless to say, the heat fuse was blown, and the connections to it were accessible on the top of the transformer - as usual though, the heat fuse wasn't replaceable. So I shorted it out, confirmed there was nothing wrong, and ordered a replacement transformer - and while waiting for it to arrive (the next day) I measured the physical size of the transformer, and compared it to those in the RS Components catalogue, which confirmed my suspicion that the transformer was massively undersized - and was only a 60W transformer.
The new one came, I fitted the transformer, did the warranty claim, and returned it to the customer. Six months or so later it came back - transformer blown again - by now it was out of warranty. I was under no doubt the reason for it's failure was due to it been too small, and contacted Sony - and after LOT'S of arguing they agreed to provide a new transformer free of charge, but wouldn't pay a warranty claim. They completely denied that the transformer was far too small, and simply said that they had had very few failures - presumably most people didn't play them loud
I was absolutely sure that the original design had called for a MUCH larger (correctly sized) transformer, and it had been replaced by a much smaller, much cheaper, one during production - but they would never admit it.
Incidentally, I kept both the old transformers, as there was nothing wrong with them.
I was hoping it might come in again, and the guy decided to replace it - and I would have kept it, and fitted a proper sized transformer from RS Components
- but I never saw it again