It is porbably entirely coincidence. As Tim said, emails are not used by admin for anything. I've run a PHPBB forum for years and never had anyone complain of this sort of thing happeing.
Did you perhaps not check the setting to keep email hidden? I seem to recall it being the default setting, but its always worth checking to be on the safe side.
I really doubt its anything to do with signing up to the forum. If your email was not hidden however, it can be viewed by people. But most bots won't bother with emails on forums like this.
I always keep a spare account purely for signing up to things online. That way my personal email is never anywhere I don't wish it.