Daisy does more damage then Tibbers does, As well as having repeated CC.
She also has more Armor and Magic resist then tibbers, all of which Scale with Ivern's AP.
The only part of Tibbers that scales with Annie's AP is his Aura.
Thematically it makes sense for Tibbers to enrage after she dies, you killed his best friend and master.
Annie isn't rewarded for dying, but players are punished for not being mindful of Tibbers.
Players are meant to think "Can I kill Annie as well as escape from/kill Tibbers?"
Tibbers enraging after death adds both a nice thematic and gameplay addition, that makes players be more mindful about something than just Annie's burst damage, which depending on the match-up and your team comp, can be forgettable.