meaning of "update forwarding xxx denied"?

[Mike Diggins]

I updated my secondary name server from BIND 9.3.5P1 to 9.4.2P2 (Solaris) 
earlier this week without any problems. Today I updated the primary. All 
is working, but I'm now logging these messages:

 	Aug 28 19:04:11 ns1 named[12157]: [ID 873579 local4.error] client
 	172.26.20.34#53281: update forwarding 'xxx.mcmaster.ca/IN' denied

This was not happening prior to the upgrade. I assume this an attempted 
dynamic update? I'm not sure what the 'forwarding' part means. I also 
don't know why it's now logging these messages, when I have:

 		category "update" { "null"; };

in my named.conf. This used to suppress these messages (failed dynamic 
updated anyway) - or is this something different? Any why don't I see any 
of these messages logged on the secondary?

BTW, we don't allow any sort of dynamic updates, but I understand that 
Windows likes to try anyway. Some clarification would be appreciated.

-Mike