From Patch Notes:
"Gameplay
When attempting to mount while in combat, an alert will now display stating "You cannot mount while in combat.""
Here's the thing: The entire way that's handled is broken.
If I'm in a group and someone ELSE gets attacked like, I should still be able to mount.
If I'm in attack range of something that's coming after me, and I wayshrine out, when I arrive at the new wayshrine, I should still be able to mount.
If I ride halfway around through the city, the thing that tried to attack me OUTSIDE the city, should have it's attack cancelled.
I'm guessing that this is on a timer rather than being based on proximity.