In my case, it appears it's because the further-down-the-list target hosts of many simultaneously spawned SSH sessions have timed out waiting for user input (such as entering a password), but SSH doesn't actually disconnect (packet type -1) until the "user" has gotten around to sending that input. It particularly manifests itself with any OS using a relatively short SSH timeout value--I have to set Expect timeout to the smallest value possible, in order to process through a full list of not that many Aruba hosts, in parallel, without getting disconnected by the ones at the end of the list. It's not as obvious with IOS, presumably due to a longer timeout value.
