At the beginning it looked like some material for the [Linuxhater blog] :-)
It happened a couple of times before, while working on my my X60 and suddenly the computer switched to shutdown mode and turned off. Uh? annoying!
Yesterday it happened again, and this time I was able to find the answer in the logs:
Sep 10 01:29:26 piscolita powersaved: WARNING (checkTemperatureStateChanges:217)
Temperature state changed to critical.
Sep 10 01:29:26 piscolita kernel: Critical temperature reached (128 C), shutting down.
Sep 10 01:29:26 piscolita shutdown: shutting down for system halt
Then googling I found out it [is a bug in the kernel] ([google groups link, prettier]), and it seems there is a patch for it. I think this is a good backport candidate for 11.0 updates.
*update*: tracking as [bug #425077]