I think this is a bug to do with formations. Being mounted takes them out of any position and opens them to circle and hamstring.
Presumably though untested a centaur/griffon that was being ridden by someone in the back row would be able to be bashed/hit in melee by enemies in the front row.
edit-  just to make it clear, this is shown by this log:
Code:
[HP:100%] [ME: 51%] [PE: 76%]
> 
Group: Caranthir's Party
(F1)  [a Taslamaran co]    (F2)  [ a spirit rider]    (F3)  [---------------]
(M1)  [---------------]    (M2)  [      Caranthir]    (M3)  [---------------]
(B1)  [---------------]    (B2)  [  the grey wolf]    (B3)  [---------------]
Characters                 Rank    Pos  Here  Follow  Prone    HP    ME    PE  
Caranthir                  Leader  M2   Yes   No      No     100%   51%   76%
The grey wolf              Member  B2   Yes   Yes     No     100%  100%  100%
Gwynnestri                 Member  ??   No    Yes     No     100%   66%  100%
A Taslamaran courser       Member  ??   No    No      No     100%  100%  100%
Hoch                       Member  ??   Yes   Yes     Yes     33%   28%   75%
A Taslamaran courser       Member  F1   Yes   No      No     100%   63%   85%
A spirit rider             Member  F2   Yes   No      No     100%    0%  100%
[HP:100%] [ME: 51%] [PE: 76%]
> glan lank
You are looking at a lanky male minotaur.
A deformed male troll and a white-eyed male deep-elf are behind him.
Overall, a lanky male minotaur has some small wounds and bruises.
[HP:100%] [ME: 51%] [PE: 76%]
> 
A spirit rider misses a lanky male minotaur.
A Taslamaran courser misses a lanky male minotaur.
A Taslamaran courser misses a lanky male minotaur.
Hoch slices a lanky male minotaur's chest, cutting a small but painful, wound.
A lanky male minotaur dodges Hoch's attack.
Hoch slices a lanky male minotaur's stomach, causing a small wound.
Hoch slices at a lanky male minotaur's chest, causing a deep gash.
A lanky male minotaur's shield blocks Hoch's attack.
A lanky male minotaur slices at Hoch's head, leaving a big scar across his face!
Hoch's shield blocks a lanky male minotaur's attack.
A lanky male minotaur slices at Hoch's chest, DISEMBOWELING him!
Hoch's shield blocks a lanky male minotaur's attack.
A lanky male minotaur strikes suddenly, slicing at Hoch's chest!  Some GUTS fall!
Hoch parries a lanky male minotaur's attack.
[HP:100%] [ME: 51%] [PE: 76%]
> 
A white-eyed male deep-elf's circle stab does UNSPEAKABLE THINGS to Hoch!