This sort of hacking (well more likely script kiddies) roughly goes as follows:
Someone discovers a security hole in certain version and brand of a website, forum, etc.
A script (little program) is written to automatically hack the forum/site etc.
But before that happened 'they' started a search program that finds all sites/forums that are vulnerable to that specific hack.
i wondered if something happened elsewhere that sent so many here
What happened is likely this:
a] A flaw was discovered in a certain SMF version.
b] They figured out this forum runs this version.
c] Accounts are fully automatically added.
It's nothing personal against TM. Likely the owners of the hack program don't even know the forum exists. It's just one of 1000s on their list.
awhile back we were getting slammed so bad with spammers, we went to every member request must be manually approved
Do you have email verification switched on? That way they have to reply to an email even before you get a registration request. Depending on the hack it may totally stop them because the email address is usually fake. Or it doesn't help at all because the hack bypasses that step.
Again just a possibility because I don't know any details.