"It was the same range of source addresses and this was with Windows server and then Office updates. What seems to be happening is that instead of the sending server reducing its window size when packets are dropped, it just keeps re-sending large windows, which are obviously being dropped at my end. The queue algorithm has no idea of this and it will be letting packets through at a rate it thinks is correct, so the flow continues even though much of the traffic is dropped. However as the traffic keeps coming, the link is totally saturated."

Translation: someone broke TCP flow control.