The most common reason is an idle timeout at the network layer, routers and firewalls will often close idle connections after a set time limit, sometimes this setting can be changed within the device or software configuration.
For example my router running the TomatoUSB by Shibby firmware has a setting under "TCP Timeout" for "established" connections. This timer is triggered to close an idle connection after the user defined number of seconds. If this value is set to say 60 seconds any FTP connection that is idle for more than 60 seconds might be disconnected.
Routers and firewalls will often silently close the connection, which means the software on either end of the connection may not be informed of the loss of connection and it isn't until they attempt to send data that they realize that the connection was reset.
Another less common reason is some devices such as routers and firewalls typically have a connection limit, when this limit is reached the oldest connection is dropped. This can be a problem when using file sharing application such as Torrents and FrostWire.
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