I reckon it's a fluke or a mass attack of BOT's trying to log in (we have lots of these every day... programs that try to register on the forum and then post advertising / spam). Luckily I've modified the software so that 95% of these bots fail, but there are spam services that employ humans to get in (and they do get in). Every day the moderators remove one or two of these spamming members.
The system counts the number of simulataneous accesses. Since around 20 - 25 is a typical maximum, everything as high as 153 should be mistrusted.
Peter