OK, it turns out that I'm just a big dork.
It seems that the Leknor script doesn't allow mod_rewrite to fix a missing trailing slash, the way a regular web browser does. When I entered the second URL, I wasn't using the trailing slash, so it was coming up with an error page rather than the phpBB2 page. When I added the trailing flash, it came up as gzipped, just the way it's supposed to.
It would appear that the big spike in traffic was simply due to a coincidental traffic increase rather than any change in phpBB2 itself; I only noticed now that my record number of users online coincides with that same time period.
So, sorry for wasting time with this non-problem