Most likely it is your natural thinning that will take place in an outdoor plant.

It is getting ready to flower.

When the entire leaf turns at the same rate, and beings to change shades in a gradual fashion, most times, this is normal.

I think its normal based on your description.

I also think it would be ok to feed it about 4 drops of Schultz Cactus fert per half gallon of water....or anything else you want. Just dont over do it.

No pics does make it hard, but based upon season, description of gradual yellowing, wilting, hanging, it sounds like a normal event and the plant will begin flowering before long.

Good luck and let us know how its going.