To ask why announce you've NC'd for a post...

I didn’t NC recently and I changed a few details (of a death that had happened and the family details)
I got reported and some not nice posters looked at previous threads and said a few things

mumsnet not nice anymore most of the time (look at the Lorrraine Thread this morning)

I find it strange when someone announces a name-change as the story will be too outing - they then go on to tell some very detailed scenario that anyone involved in the tale would instantly recognise!

I think OP is asking why they announce they've NC'd not why they've NC'd.
I've often seen posters call out other posters as being fake, journalists or trolls when it's a first post under a name "Oh interesting first post 🤔". People often seem cynical about a poster if it's their first post, so people say "I've name changed for this post" to explain why they have no other posts under the new name.

Also posters (myself included) very often change a couple of minor details that aren't material/relevant to the actual issue to make it less identifiable. But then maybe they need to post something where actually, that detail is now relevant, so they NC rather than have posters going "AHA but last year you said your DD was 12 so how can she be 15 now".

Because if you put one post about being a nurse

One post about living in Norwich

One post about what to wear on your 40th birthday (that also happens to mention you're 5 foot 10 and a redhead, size 16)

Then it becomes really easy to work out who you might be.

If you then want to post about how your sister has really pissed you off, your sex life or how you've got chronic diarrhoea you might not want that linked to all your identifiable info.

For me, it’s when I’m posting something I’ve shared / ranted about in real life. So anyone who knows me would know that was me. I don’t really want that linking with any posts about medical conditions, work situations, family problems, children’s medical details etc.

Absolutely the fear of people searching previous posts and putting two and two together especially if it's sensitive.
My username gives everyone a head start!
I nearly always post to help other people but if I was needing help myself and it gave another big chunk of information I'd probably name change.

And they ‘announce’ it to pre-empt the people who will look up the user name, see it’s the first post from that name, and assume they are a new (and not genuine) MN user

For me it's when I think I've posted something that if you knew me and linked it to a few other of my posts you would be able to work out who I am or have suspicions. So if I NC I announce it so people don't think I'm someone who's new and starting trouble.

Because you might be about to post something you know is massively identifiable and you don't want that to mean all your previous threads can then be linked and people you know can find out loads more about that wasn't previously identifiable

Some people on here are truly bonkers and will go through a poster’s previous history and challenge them if they feel stories aren’t consistent or will make comments about it being a dodgy first post if they can’t see previous posts. Personally I think there are a lot of people here who need to get a life and remember it’s just an internet forum but I do understand why some people would name change and state it to let others know it’s not a first post.

It's usually either because something posted is potentially identifiable and the poster doesn't want to have her entire posting history searchable to a person she knows in real life who just figured out who she is, or she'd like answers to the question posted without someone dragging up something she posted 5 years ago which has nothing to do with their current thread.
I'm sure there are other possible reasons but that's how I usually see it.

Previous posting history will invariably be looked at by someone. Thus posts like, 'have you posted about this before' and 'you stated in your post from 2012 x which contradicts y' .

You might have previous posts and together they could be identifiable.

You might have lied before and dont want to get caught or you might be lying now.

You dont want the same answers you got in your last post about this, but seem unable to realise that you will still get the same answers.

It’s because people can search your previous posts and link together details of your life which potentially could ‘out’ who you are.