I know what would have caused this, although I didn't realise at the time that the forum was affected. In short, our
backup system didn't work as smoothly as I'd initially envisioned. The backups have been done hourly rather than minutely for some time, as it turns out that repacking a git repository with commits made every minute for several weeks takes up a
lot of memory.
Even with the backups running hourly, the repository gets pretty big after a while, and the server ran out of memory a few hours ago while running a backup job. I haven't exactly solved the problem as such, but I did archive the old backup repository and create a new fresh one, so it won't happen again for a few months (during which time we'll hopefully come up with a better backup strategy).