This just means that the VX9000 is getting more adoption requests than it can handle with the available resources. So, the controller throttles the new requests till it can complete the ones it has already signed up for. Once it completes some of the current adoptions, it starts accepting more requests.
The throttled APs will wait for some time and then attempt adoption again.
So nothing to worry, unless you are noticing that the adoption never completes for any of the throttled APs.