I notice "calling the vet" has not been mentioned
The horse "trips a lot" could mean any number of things. A crummy farrier, eventhough nearly everyone is "in love with their farrier".
It could mean early stages of EPM.
26 or not, unless there is some underlying cause you're not aware of, or the farrier is just stupid, 26 year old horses don't just trip. I have taken two horses to ages 29 & 27, neither of them ever tripped just because their old. My current Arab is 26 has an injured vertebra, when he starts tripping and/or leaving snake trails with his hooves in the dirt, I know he needs to see the chiropractor.
Your horse moaning when he was down could most certainly mean he had a colic attack and that attack could have been brought on by some form of ulcers - foregut or hindgut.
Horses don't moan for no reason - not only are they very stoic but they are prey animals and to show weakness oftentimes means certain death in the wilds ---- it's in their gene pool no matter how many years we've had them domesticated.
I have a 25 yo TWH who's been with me since a coming 3 yo. He has equine metabolic syndrome and hind gut ulcers. I almost lost him to his first-ever colic last March. Thankfully when he went down, it was in his stall and I was lucky enough to get the vet out in less than an hour - at 7:00 PM.
Even in the seriousness of that major colic, my horse never moaned - never.
For all outward appearances, it doesn't matter how many times he gets ridden and how good he looks and acts --- he did not go down and moan "just because".
I will stick my neck out and say the moaning was caused by something internal, and the fact that he tripped and went down may be totally un-related to his other episodes of tripping. It could be the pain grabbed him to the point he just couldn't stand up anymore.
Please call the vet.