Shattered Kingdoms

[HP:100%] [ME:100%] [PE: 73%]
>
A failed squire dodges a centaur charioteer's attack.
A centaur charioteer quickly jumps out of the way as a failed squire kicks dirt into the air.
A failed squire misses a centaur charioteer.
A failed squire misses a centaur charioteer.
You stab a failed squire's chest, ripping open his stomach!
Overall, a failed squire has quite a few wounds.

[HP:100%] [ME:100%] [PE: 73%]
> gla squire
You are looking at a failed squire.
His chest is bleeding.
Overall, a failed squire has quite a few wounds.

[HP:100%] [ME:100%] [PE: 73%]
>
A centaur charioteer misses a failed squire.
A failed squire slashes a centaur charioteer's forehead, causing a small wound.
A failed squire slashes a centaur charioteer's chest, cutting a small but painful, wound.
You strike suddenly, stabbing at a failed squire's chest! Some GUTS fall!
Overall, a failed squire has quite a few wounds.

[HP:100%] [ME:100%] [PE: 73%]
> trip
Someone is in your way.

[HP:100%] [ME:100%] [PE: 73%]
>
A centaur charioteer misses a failed squire.
A failed squire misses a centaur charioteer.
A failed squire misses a centaur charioteer.
You miss a failed squire.
Overall, a failed squire has quite a few wounds.

[HP:100%] [ME:100%] [PE: 73%]

Why exactly can't I trip him?

The squire was in melee combat with another NPC. For some reason, the game is assuming that other NPC is 'in the way' apparently. I don't know if it's a bug or not, however, but I suppose that's why it happened.