I believe there is a limit on the amount of attacks a character can do during its turn (in order to prevent some infinite loops, I guess, like an immune attacker hitting an immune target indefinitely), and sometimes you hit it, even though you might have gone on otherwise (an infamous example is Garrosh’s mak’gora to the death vs Nefarian’s first stage) and eventually killed the target.
As for big trees, doesn’t it say that they are upgraded each time the Corrupted Ancient takes damage? I believe that’s the way it worked.