I ctrl-f'd the bug list for "ambush" and I didn't find anything like this, I apologize if I missed it.
I was ambushed by an ambusher party at the edge of the map. However, I checked and no dwarf was around the revealed ambushers. In fact, there was way up or down the mountain from the ambushers' position, aside from the map edge.
Then I noticed that among the horses was a horse foal. "That's odd, I didn't think invaders could mount young animals." Although wild horses did spawn on the map, these ambushers were far up in the mountains where no wild horse tread. I looked it up in the unit list and the foal was listed as "hostile." That's odd.
Then I check the announcement log. Just before the "An Ambush! Curse them!" message, there's an announcement about a horse giving birth to a horse foal that I didn't notice before, despite checking the log shortly before the ambush warning.
Given all of the above, I deduced that in one of the previous raids, possibly even one of the very first ones, one of the ambushing parties became trapped on the mountain almost immediately due to the massive mining work I've been doing. They sat there unseen for years, the horses got bored, and now one of them has just given birth. Because the game wasn't expecting ambushers to last this long at all, let alone without being discovered, it didn't handle this optimally.
I don't have a very recent save from prior to the ambush discovery, but I do have a save from (assumingly) the exact frame in which the ambush paused the game, if that would help at all.