An expression found in a short story by contemporary writer Han Shaogong
Han Shaogong uses a lot of topolect phrases in his writings. This could be one of those.

Apparently 赶马驭树的人 means "people who pass by" in context. 赶马驭树 doesn't make literal sense, especially the 驭树 part, but a lot of things in language make no literal sense. 

A similar, but more widely know case could be 偷鸡摸狗. It's just a general term of thieving. You don't need to steal a hen and touch a dog to be called a 偷鸡摸狗的人.

The story background is at Jiangxi, Han Shaogong himself is from Hunan. Perhaps someone from those regions could explain it a bit more.