There's no way of knowing what's causing it..........could be from a heart-failure type thing or a liver problem, cancer, etc. I had a hen who had this and I would drain her about ever 3-4 months over about 7 years! I watched the avian vet do it the first time, and was just very careful. I had good equipment to work with. But......if you are afraid......what you could do is get a big gauge needle (like from Tractor Supply....maybe a 16 or 18 gauge) and poke her abdomen in a couple places. I would always put the needle in over by the left leg, where her abdomen met, since that's out of the way of other organs. But she would continue to drain by gravity, even after I'd drain a lot out. So I'm thinking if you could just gently put a couple needle holes in her, she could drain herself.
The liquid I got out of this hen was always clear. But when I did it to another hen, it was green.......which is much worse than clear.
You might pull up on the skin in order to make the holes, so as to avoid hitting anything important underneath.