The things that come down might be a trimmed branch, dead wood or an entire tree that might be dead or alive. The cuts are made with sharp saws very close to the climber and climbing rope. The lack of attention to a tiny detail while aloft can change or end a climber's life. Many climbers are killed. A professional climber truly lives life near the edge on a daily basis.
Good climbers are rare individuals.
That's the bad news. The good news is that you don't have to climb to be an arborist. Trees get sick just like humans. It takes a sharp person to figure out what a tree needs to get better. There are many clues to read, just like a detective does.