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!