More details:
On my server the "Sender Reputation" feature is enabled with the Threshold set to 0. The block action is to add the IP to the block list for 24 hours.
Obviously with the threshold set to 0, default it 7, lots of IPs get added to the IP block list. When the TMG Managed Control service detects that IPs have been added to the list the email policy is reapplied to remove the IPs from the list.