Why did Riot buff trist

It wasn't a buff. General Attack speed per level reduced to 2.5% from 4%. Level 18 is a -27 from levels, -7 with Rapid fire (which is now 5 seconds long)

Rapid Fire Attack speed increased to 30 / 50 / 70 / 90 / 110% from 30 / 45 / 60 / 75 / 90%. Duration reduced to 5 seconds from 7 seconds.

It's a "buff" around mid game if you max Q first at level 9 (only 7~ more attack speed with rapid fire, -13 without) but still reduced duration.