That's the way mine usually goes. Despite fine weather.

I think it has something to do with the length of the day.

Plus they're getting old.

G...I totally agree with you...about the diminishing light. I really think temps are over-rated (except frost, etc).

I never thought of shortening day length to be a factor. Because in the tropics day length are around 12 hrs. Unless it is part of the genetics of tomato plant requiring longer days or preferring it.

But , I thing the length of growing season and cooling off signals something to the plants. For this reason some suggest that near maturity fruits ripen up faster than before.

The fall weather is almost everywhere in the north half of us.
Some places have already seen their first frost.
In our location at PNW, our high - low is forecasted as 59F-46F. Not quite a frost but getting there.

I have been topping new growth and flowers as of early August but I will have some green tomatoes that will little chance to ripen.

I checked the temperature map of USA@ 5:10 AM.
As you see, it is getting BLUE all over.

makes me feel blessed...as I read that chart north to south!

My tomatoes are worn out. The ones producing are making smaller tomatoes. Bugs and rain and drought and heat have taken a toll. The only thing still going like mad is Matt's Wild Cherry - foliage is pretty little tomatoes are bright red. I predict I will have millions of them next year.

Here is a link that might be useful: investigate and plug in your city