The heat may have set off their resting cycle. With my Runners, they laid daily for over a year, then took two or three months off, then laid for about ten months, 3 months off, and so while they've slowed down individually, we have enough ducks that we always have more than enough eggs except for the tail end of their vacation.
From what I see with my flock the resting cycle may start with a change in the weather, or with going broody. Then after that, molt. Then about six to eight weeks later, they are laying again.
I am happy for them to rest - I get concerned about prolapses and other health problems from laying too too much.