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When you upload an image, it is put into a queue to sync it up with the other mirrors as soon as possible. However if many large images are uploaded at once or the queue fails, the length of the queue will grow.

Then you get what petopeto said.

petopeto said:
It's just a feedback loop when mirroring falls too far behind: everyone starts bashing on the main server to get unmirrored posts, which slows the mirroring down further...