if the horse isn't stepping forward with some energy they can be more prone to tripping. a better rider may have the horse more focussed, and more forward, thus not trip as much.
My friend had a nice Appy who was a terrible tripper. I rode him a fair amount, and forgive me for being a bit immodest, but I really worked on making him step forward , lift and carry his own head (not lean on the rein) and he improved quite a bit. STill, he tripped some no matter what, and his hock action was obviously "differeent" , so I think to a certain extent, he couldn't help it. He tripped twice while I was riding and went down on his knees. I never could totally trust him.
Now I am riding a hrose that , I swear, NEVER trips. it's is so very nice. no matter how messy the trail, he doesn't trip.