Sivir Spell Shield Bug?

I don't know if this is a bug or is intended, but was playing an ARAM as Alistar and the enemy team had a Sivir who waited to use her spell shield against me. Totally okay with this if it weren't for the fact I feel she's blocking two abilities at once. When I would go to do my W>Q combo, she would E instantly. Now, I'm not a design major or anything in gaming, but I was under the impression spell shields blocked ONE ability. So if she blocked the W, she should be knocked up by the Q. If she took the W, she would be displaced at least, though I don't think she'd be able to block Q since it's like a stun (I believe, though not %100 on abilities).

Am I missing something here or is this a bug? Or is this a unique interaction with Alistar?