There are three instances when a connection could be closed from our server side:
Connection is killed if no data is sent for 15 seconds
When our load balancer restarts its workers. It gives some time for existing connections to finish (which is 1 hour if I'm not wrong). Then if there still are alive connections, they're killed
The exit node goes offline temporarily
If any of these affect your work flow you can reach out to us and we will see how we can help you.