Certain threads generate enormous amounts of activity, and the reddit infrastructure will automatically place those threads into a temporary 'Read Only Mode'. This effectively ends the thread when it revolves around rapid updates, so redditors normally start 'Incident Thread 2', then 3, and so on.

During the Boston Marathon bombing, there were, if I remember correctly, over a dozen threads that continued the original conversation, simply because each of the previous threads kept getting hammered by people posting comments.