Do you pay for google apps? If so, go into the help section - there's a 1800# to call for tech support. Depending on what the problem is, they may or may not be able to fix it (if it's not a google problem), but I would be shocked if they weren't able to tell you why this is happening. (if you don't pay for google apps, it might be worth paying, for the tech support).
I am not too familiar with Google Apps mailing but if you have access to the mail logs that would be a good way to backtrack what happened.
The logs usually contain server responses to your sends with instructions/link on how to be "safelisted" or at least a reason of why you were blocked. Determine the domains and go from there.
http://senderbase.org/senderbase_queries/rep_lookup
http://www.mxtoolbox.com/blacklists.aspx
Also make sure your email list is double-opt-in so that malicious people cannot sign up other people for email.