This is probably caused by cross-references pointing to those other documents that get opened. Are there any in your document? (CS3 didn't have cross-references.) Open the Cross-References panel (Window > Type and Tables > Cross-References) and check if there are any in the panel.
Might also be the system preference to reopen windows that were open when an application is closed.
There's nothing in the cross references panel, I'll keep an eye on this in future though as I was unaware of it - and nothing in system prefs also unfortunately.
It doesn't seem to happen all the time though, so for example, when I opened a file this morning that i had worked previously with no such issues - lets call it 'september.indd'. As soon september.indd loads, a new document that i didn't ask to open (lets call it 'august.indd') opens up repeatedly - this morning I ended up with september.indd open and 12 x august.indd (all showing as converted). As i tried to close the copies of august.indd more opened up. I ended up just force quitting indesign - and on restarting I got september.indd opening fine with 12 copies of untitled.indd, which all closed fine and I had no more problems with september.indd.
The whole things abit unpredictable, so i haven't been able to replicate it since my original post but i will keep an eye on it against your suggestions for when it happens again in the next day or so.