That sounds similar to some reports earlier this year.
viewtopic.php?f=6&t=47669 There didn't appear to be any confirmed explanation and it went away after a while. One might speculate on someone testing a spam posting script, your gallery URL being posted on some hot social networking site, or some other weird stuff. Hard to say, I experienced an increase in hits for the first few days in November, but they weren't significantly higher (certainly not 400 a day on one gallery), and I had posted some new images.
For sure, there is no shortage of mystery on this site!
EDIT: And now re-reading, I see you were in that earlier thread, so I guess I haven't suggested anything new. I had a large increase in hits in May and June, maybe twice normal, but things have seemed more normal since.
DaveT