Exponentially slowing download syndrome

[Lanigiro]

Every so often, one or even several of my downloads will develop what I call "exponentially slowing download syndrome". When that happens, the displayed speed starts dropping, often initially like a rock, but the lower the speed gets the lower the rate of decrease. The countdown also becomes a countup, indicating that it's getting slower fast enough that it will never complete the download, but asymptotically approach some percentage of the file lower than 100.

What causes this, and is there some button I can push to immediately fix it? Something in the right click menu that will arrest the decline in speed, if not restore the original speed it had before it became affected?

The odd thing is, I can think of no reason why any client would be designed to apply such a peculiar time-sequence of throttling to a download (or to an upload, if this is caused by a policy enacted at the other end). I can see the speed making step-function changes, say when some uploader's bandwidth goes from being divided N ways to being divided N+1 ways, and indeed I often see such step-function changes. But what would result in either end applying a throttling to a particular transfer that seems to follow an exponential decay curve over time? I just can't see any legitimate purpose that would be served by including such a "feature" in a peer.