How can you hit master yi WHEN HE ALREADY CASTED HIS Q ???

i was in alpha

You can still be hit by DoTs and leashed abilities, the same as any other untargetability (except for stasis.)

Were they AOE abilities?

They could have attacked you before you hit your q and the attack still followed through. An all too common thing you'll notice as Trist, when you W and an ability is chasing you down the whole way.

It's bugged right now; AOE abilities are very often hitting Yi during his Alpha when they really shouldn't be.