She's receiving buffs because lethality builds aren't optimal on her. She's an objective taker as well as a reset bot. She has the extreme range late game to not need lethality to smash squishies. She just build crit and last whisper and melts anyone with just a little more time, and can continue the chase. She also has the best self peel in the game.
So, they're buffing her attack speed to bring her DPS up to par with the lethality abusers. Two seconds added on to rapid fire, which at 1.5-2.5 attack speed, is actually massivw. She will be better at taking objectives and her window of power has now been extended. I personally find her to be a sleeper because of how her laning phase is structured and how her late game is ridiculously broken. I think we will be seeing a lot of Tristana, especially once this buff rolls out and then they eventually scale down lethality a bit. Hopefully we won't see a repeat of Season 5 (4?) Tristana.