You can no longer comment on this thread as it was closed due to no activity for a month.
There are several things that could have done it, and I don't know which happend to it, but I'd bet money that is because of a cracked shield.
Some of the things that could have caused it:
1. extreme abrupt temperature change (thermal shock)
2. dropping it
3. incompetent repairman cleaning the sensor
4. internal stress fracture
5. a micrometeorite hit it while you were changing lenses
6. you pissed off The Sensor Fairy