Yes, that's normal. The Skeleton Surprise event was unlocked when you defeated Bonemass.
Just bear in mind that events are directed at you and not at your 'base'. That means the chance for a Skeleton Surprise event can occur anywhere, in any biome and only require you to remain in the vicinity of three structures you have built.
It's a clever if not devious mechanic employed by the dev's to encourage you to get out more and explore the world rather than sit around in your base ;)
https://valheim.fandom.com/wiki/Events