Twenty dead tamed boar in a pen, one dead tamed boar, Frank, killed while roaming your base. Twenty-one undamaged domestic bee hives, two damaged domestic bee hives, and one destroyed domestic bee hive. No other damage discovered. That could be a civil war between domestic creatures.
Questions I would ask:

1) Are any of the bee hives close enough to the pen that the boar inside could aggro the bees?

2) Presumably, the twenty dead tamed boar inside the pen dropped all their boar meat and leather scraps inside the pen. How close to the boar pen were Frank's drops?

What I am thinking here is that a civil war between domestic creatures would require one of the boar to aggro the bees. If the boar in the pen were not close enough to the hives to aggro the bees, why did the bees attack them? If Frank stirred up the bees, he could have led them to the pen. If he died some distance from the pen, that might indicate that he did not. Perhaps an unknown third party is responsible for all the deaths.

If I were you, I would ensure that all my domestic bee hives were located where no domestic boar could aggro the bees. If you allowed another Frank, your apiary would have to be some height off the ground. That would require a system of ladders or stairs which I viking could only accessed by jumping. Frank would be able to run under the hives, but the vertical distance would still put him out of aggro range.

With this precaution in place, should you lose more boar and hives, a third party is almost certainly at work.