I have had my maple bonsai for about three years now. Recently, several of the main branches have started to dry out and die. This often begins near the leaves, which turn brown, and then the entire branch dies within a couple of days.
I regularly water the plant and keep it out of direct sunlight during the summer. (Two years ago, all the leaves died after just one day of direct sunlight.) The leaves fall naturally, and I have not pruned it myself.
What could be the root cause of my recurring branch loss? Any advice would be appreciated!

The first thing that comes to my mind is repotting stress. But you did not mention you have repotted it. Maybe it was repotted just before you got it and that corse substrate make it take longer to establish itself?

Thank you very much for your reply!
I'm not sure if the issue is related to repotting...
Starting to dry from the tips toward the main trunk. Many branches continue to die, even though that part looks alive and green today. 
I've added a new picture of another branch that died this week.

I am not sure either . When was the tree repotted last time? Do you have any pictures from when you got it three years ago?