No idea, it was just there - no mods or anything that would change it. It's not the normal layout for that floor, it looks like it was randomly randomized (lol), it just pulled an ice fortress layout instead of a forest one. Perfectly normal forest floor otherwise, same enemies, drops etc, no weird glitches or anything. The grubs might have been a bit chilly, though.
Yep, would normally be a forest, just randomly had an ice fortress background for the day. No gameplay effect, I'm just easily amused.