I've got Autumn Joy Sedum all over and the oldest ones sprawled out and laid flat. I've never watered them -- let nature do it -- and I didn't change that this year, maybe that was the issue. The ones that sprawled are about 7 years old. They younger ones didn't sprawl, but they weren't as big either. They are still blooming beautifully, but the center is just stalks splayed out.
Is this time to dive or just strange weather this year?
I don't know about the watering but my daughter has two autumn joy sedum's one gets full sun the other part sun and the one in part sun always sprawls - any chance this plant is getting too much shade?
Mine started doing that after it got very mature. I divided it and it looked much better. If you really like it it's worth trimming way back and dividing severely. Give some of it away if you don't want/need all of it. I don't think watering has anything to do with it.