This article describes how Collaboration Services picks a domain controller (DC) to connect to for active directory (AD) information.
The Global Catalog (GC) server contains only a partial attribute set of all forest's objects, so not all attributes can be retrieved by connecting to GC. That is why, for each domain, Collaboration Services chooses a DC to connect to, and the information about "the chosen ones" is logged into the Collaboration Services event log when the synchronization service first starts.
Collaboration Services intentionally uses a single DC (in other words, does not use server-less bind), not to round-robin between domain controllers - otherwise it would experience problems with replication.
If the selected DC goes down, Collaboration Services would automatically switch to another one after some time of unsuccessful attempts to reconnect to it. By default, all these activities are fully automated. You can also specify which DCs to connect to for each domain, using the Collaboration Services properties.