From the white paper:
"It is likely custodians will demonstrate they hold a substantial quantity of NuShares so that shareholders know custodians' interests align with their own. It is likely that at some point a custodian will disappoint shareholders, but the damage they can cause is limited to the value of the grant they have already received."
It's like in the real world: if you delegate power, try to be sure to grant it to the right person.
Parking needs voting from holders of NuShares as far as I got it.
As far as I understand just the interest rate gets voted for. The higher the interest rate the more people will park.
And I think there are custodians which build up a reputation or proof that they have an interest in doing like they are told (like having a big stake in nushares).