Most likely cause is that the exhaust clamps weren't tight enough or that copper grease had been used on the joints to make them go together more easily and stop them siezing up; best just assembled dry, exhaust sealant can make them damn difficult to take apart.
Another possible is that the nuts on the studs at the exhaust ports have slackened off (or even a stud has sheared) allowing the header to turn pushing the tailpipe out of position.
Could also be a splitting mounting bobbin under the silencer.
Shouldn't be a big problem to sort it out.
