Need to understand parallel query execution error
Few months ago, I faced similar situation wherein the MAXDOP setting was default and a run away query exhausted all the worker threads.

As Remus pointed out this is called worker thread starvation.

There will be a memory dump created on your server when this condition occured.

If you are on 2008R2+SP1 and up then sys.dm_server_memory_dumps will give you the dump file location as well.

Now back to the problem :

There is 1 scheduler monitor thread per NUMA node and since you have 2 NUMA nodes there will be 2 scheduler monitor threads which are responsible for health checking of all the schedulers every 60 secs for that particular NUMA node while making sure that the scheduler is stuck or not.

Each time a new work request is pulled from the schedulers worker queue, the work processes counter is incremented. So if the scheduler has work request queued and has not processes one of the work requests in 60 sec the scheduler is consider stuck.

Due to a run-away query or extensive parallelism, there arises a condition of worker threads begin exhausted as all the threads are occupied by that single run-away query or excessive prolonged blocking and no work can be done unless that offending process gets killed.

Your best bet is to first tune your Max Degree of Parallelism setting. Default of 0  means SQL Server can use all available CPU’s for parallel processing and there by exhausting all the worker threads. 

There are many reasons that can lead to exhaustion of worker threads :


Extensive long blocking chains causing SQL Server to run out of worker threads
Extensive parallelism also leading to exhaustion of worker threads
Extensive wait for any type of "lock" - spinlocks, latches. An orphaned spinlock is an example.


Refer to my answer here that will show you how you can calculate MAXDOP value for your server instance.

Also, highly recommend you to start collecting Wait stats information about your database server instance.