I certainly haven't seen it that often but I have seen it. There can be a few explanations.
Since we have to assume the server is up either the server isn't getting the request or we're not getting the response. This could be happening because the server is overlaoded or the network connection to it is. It could also be happening on our end. Most home routers use some form of NAT (network address translation). Depending on your router, it will support a given number of NAT sessions at once. It might be 512 or less. Better routers will have 1500+. Enterprise grade routers may have 4k+.
When you run out of NAT sessions it can act like a bad network. Randomly dropped packets, etc. There are a couple of reasons that often explain NAT session exhaustion:
You're simply using too many connections. BitTorrent clients are particularly susceptible to this. You might be attempting 100-300 connections per second per torrent; or
The timeout value on your router is simply too high so connections aren't being made available quick enough.
Or you might have a few computers on your home network (or even corporate network) that are simply trying to do more things than your hardware (or how you've configured it) can handle it.
My own router I bought because it supports 1500 and I was hitting that on occasion. I changed my settings (in both the router and on utorrent) and it rarely gets above 300 now.