On June 28th, many TMG admins started seeing something odd going on with their TMG servers: the TMG services stopped unexpectedly, especially when using SSL publishing. This generates an error message in the Application Events log on the TMG server, notifying you that the firewall service has stopped. The solution is to apply the TMG SP2 rollup2. The Forefront TMG team reported on this soon after the problems manifested:
http://blogs.technet.com/b/isablog/archive/2012/06/29/tmg-services-stopping-unexpectedly.aspx