Whn in trot, my four year old (overweight) cob appears to trip. His back feet seem to catch in the ground or his ankles go weak and he collapses through them. It's very hard to describe because I'm on top and I've only seen him do it once on the lunge.
I know he's still finding his feet being a youngster and all.
His legs click a fair bit so I walk him lots in corners/curves until the clicks lessen before moving on.
It's like he twists his ankles. He doesn't limp afterwards just hold the bad foot up a while then carries on.