Are you near the swamp? possibly a blob or oozer came over & released a poison attack on the boar through the wall perhaps?

Maybe a Campfire was placed & the boar ran over it or freaked out & burnt to death?

Did you stay within render distance of the boars during taming? (I ask because sometimes the boar will render in before structures will from a distance, and hence the boars would basically just walk ''through'' the wall before it loads in when you get closer to it) Hence why it is a good idea to simply stay closer to the boars, but not too close as to startle them.

Was the enclosure adequately sealed off from the outside? No gaps in the walls etc?

Did the bat raid happen recently? Boar a prime targets in a bat raid.

Only other thing I can think of is there might have been a world generated beehive in the building you put them in, and they wandered too close to it, and died.