Why does the flight controls check come before arming the autobrake on the A320?
Both are in taxiing SOP.

The reason is in case the spoilers were accidentally left extended on the flight before (or during a maintenance check before the flight by the line engineers).

If that's the case, then pressing MAX while taxiing before checking the flight controls (spoilers included) will engage the full RTO brakes and surprise the crew, injure a cabin crew member, and/or cause an accident (a ground vehicle thinking you won't stop and is aiming to pass behind but then you stop in an instant).

Source: Operational Liaison Meeting Taxi Procedures (PDF page 7).

I'm guessing someone has done it before.