Active Directory Domain Controllers only support teamed network cards in a “Fail on Fault” configuration and not in a load balanced configuration.

To quote Microsoft from the Windows Server System Reference Architecture (WSSRA):

“The second way to protect against a network failure is to configure each domain controller with two separate network cards. Each card is wired to redundant network switches and the cards are configured as a fault-tolerant team. The server would have one IP address, but in the event of network switch or card failure, the server will continue to operate using the additional connection. Note that this configuration can only be used for fault-tolerant capabilities, not load balancing. At this time, Microsoft does not support load balanced network teams on domain controllers due to potential data corruption issues. There is an increase in hardware costs, due to the redundant network cards, cables, and switches that are needed; there is also an additional management cost. In addition, this configuration would not protect against a failure at the router level.”

So if you have teamed your network cards in a non supported configuration – this  could be a reason.