Hi, I run a forum like this one, my most online is 595 and my "online at the moment" is 203. This doesn't mean we've become less active - in my case the 595 was only set a week ago (meaning activity is actually picking up). The 595 isn't a good figure though because it could represent a spike for various reasons. We (my forum) tracked one such spike down to being mentioned on a prominent blog (so for one day and one day only a lot of people visited). Also spiders and other bots crawl these sites, sometimes they all match up with their times so you can get a lot online at once.
Or, someone could have been DDoS'ing the site in which case thousands of hits would be recorded at once (seen that on another such forum, they recorded 12,394 online on a site that usually gets 10 a day or something).
In short, the world of forum statistics doesn't mean much at all. A good analogy is trains: imagine if you will, that for one night and one night only, Michael Jackson came back to life for a gig at the NEC. Birmingham International may well record 5 billion passengers in one day, but they wouldn't use that as a statistic because that's just one day, come the next day they'd be back down to their average again.
Anyway, probably either:
1. A day of a major crisis on the railways caused a lot of people to come online (feasible)
2. The site was mentioned in a popular blog or website
3. Glitchy crawlers/spiders/other bots/ spam bots
What it doesn't mean is the site was more active / is now less active.