(I don't recall ever being taught the rules of punctuation/grammer/capitalizing in school -- fiction-writing wasn't ever a major focus, as I recall -- so I've mostly gone by what I read in books, and using that style. I guess you could say osmosis and a bit of logical guesswork taught me things like that punctuation goes inside the quotes (at least in US-based publications), etc.)
Normally, I'd write a sentence with dialogue like this.
"Hello," they said.
The entire sentence is hello-they-said. First word is 'hello', so it's capitalized. Since 'they' is not the first word and not a proper-name, it's not capitalized. Thus, it made sense to me that when the order is rearranged, the capitalizing is also rearranged:
They said, "hello."
I'm pretty sure this is a pattern I've read plenty, 'cause I had to have gotten the impression from somewhere that this is alright. It's also why/how I learned that when you've got a tag in the middle, capitalizing is still applied as an overall:
"Yesterday," she said, "it was sunny."
First, 'yesterday' is the first word. Second, the actual sentence -- 'yesterday it was sunny' is an entire sentence and the tag 'she said' is just inserted. Similar to the way if I had [ed: hi there] in the middle, it inserts, not halts the sentence and forces a new one. It's like a paren.
In my mind, if I've got a sentence like the following:
"Yesterday it was sunny," she said. "We napped."
...then the "we" gets capitalized because it's a new sentence; if it hadn't been, then it'd be a comma after 'said', not a period, and there'd need to be some kind of a tag -- ie, 'and', 'but', etc -- before 'we' to indicate there was more to the first sentence.
I'm not sure whether this is a house-style thing or just something I've completely misread/ignored all these years.