I think the problem started with the words "Some of you may know me from a previous life where I had a different username, in which case "Hi" - and sssshhhh we don't talk about that any more." in the OP.
Now, obviously Tyepcast couldn't have known this, being newish and all (Okay, returning, but a lot changes in a couple of years), but occasionally, when a long-time troll and troublemaker comes back they will make such a reference, in an attempt to get around their banning but still let people know who they were despite not mentioning the name that they were banned with. It has happened before, and while I don't condone name-calling, I think what happened here was that people either misconstrued this as an old trouble-maker coming back with a new IP to cause new trouble in a flamboyant style, or a person hitherto unknown on Mumsnet trying to set themselves up as one of the new inner circle (if that sort of thing genuinely existed here).
Normally, when I join a new forum, I lurk for a few hours, get the tone of the site, and then either introduce myself appropriately, or realise that introductions are not needed and just post in threads. Given that there was a thread in existence for the purpose of promoting blogs, most new people have given themselves a small introduction there, and not felt the need to start a thread of their very own.
To clarify, I don't think there's anything wrong with starting a new thread to introduce yourself, but the unfortunate combination of what appeared to be an attention-seeking thread from a new person who made references to having been around before (which, unbeknownst to Typecast means "troll" to regular posters) meant that people got the wrong end of the stick and either assumed that Typecast was a returning troublemaker to be driven off, or someone with a bit of a superiority complex who was going to be annoying to have around.
In essence, generally, when you join a new forum, you don't do a big "TaDaaa!" unless you are someone who most people on the forum would have reason to know. I don't know of any other internet forum where such an entrance is the done thing, whatever the common link that brought people to it. The reaction was excessive, and the tone considerably more hostile than necessary, but its negative slant was, in my opinion, somewhat justified.