The instinct to add one more sentence of justification usually comes from a good place, wanting to be understood, wanting to preempt objections. Past a certain point, it stops helping and starts working against you.
Explaining gives someone the information they need to understand a decision or a request. Overexplaining keeps going past that point, usually because the speaker is managing their own anxiety about being questioned rather than actually adding useful information for the listener. The listener can usually feel the difference, even without being able to name it, it reads as a lack of confidence, even when the underlying reasoning is completely sound.
"So the reason I'm asking for the extra day is, well, it's not that I didn't plan well, it's more that some things came up that I didn't totally anticipate, and also the scope kind of grew a little in ways that weren't really my fault, and I know deadlines matter so I don't want this to seem like an excuse, but..."
"I need one more day. Scope grew after we found an edge case in testing, happy to walk through it if useful."
The second version conveys the same core information, more time is needed, there's a legitimate reason, in a fraction of the words, and it reads as considerably more confident, even though it's less defensive on its face. The first version, despite saying more, actually communicates less, because the excess words read as anxiety rather than information.
Every sentence past the point of understanding isn't adding clarity. It's subtracting confidence.
A useful test while speaking or writing: after you've stated the core fact and the one-sentence reason, ask whether the next sentence is adding new information or just restating the same point with more anxiety attached. If it's the latter, that's the sentence to cut.
This shows up especially often in apologies and requests, where the instinct to soften a hard message balloons into paragraphs of justification that ultimately undercut the message itself. A short, clear statement, followed by an offer to go deeper if wanted, "happy to explain further if useful", respects the other person's time and reads as considerably more assured than trying to preemptively answer every possible objection in one breath.