Need more info to help...such as where do you live? please add details after your user name as we have no idea of your climate etc. Does it actually get sun on it as I see a covered walkway?

Cactus soil isn't good for DR's, they need perfect drainage which it doesn't support. The mix looks way too organic so my guess is that it's got root rot issues due to water retention. Plus the pot looks rather large and it's potted too low down.

How long has it been dropping leaves? It may be going into its dormant stage, but without location, climate etc, we have no idea. How warm is the room and has it been housebound all along?

Has it ever been fed, if so with what and how frequently? How long have you had it?

Gill