It took a while to find this, but here is what happens in the orc hive with a flying mount. It is slightly different from the earthquake problem, but the conditions are the same.
Quote:
The platform collapses, sending you into the chasm!
A silver-winged pegasus stops flying.
You dismount from a silver-winged pegasus.
You lose your concentration on spirit sight.
You can no longer see the spirits.
You lose your concentration on spirit aura.
Your spirit companions leave you.
You have been KILLED!!
The pegasus for some reason stopped flying once the platform collapsed. You don't see any strange behavior like that where earthquake is concerned, but it is still a problem with the coding not realizing you are on a flying mount.