FWIW, my *GUESS* is that this wasn't planned. Somebody the other day said that this kind of rocket launcher can be set to automaticaly fire on anything passing overhead. That sounds to me quite plausible. So my guess is that somebody set it up on automatic, and took a break when they shouldn't have, or that they were assigned to do something else, so the left the launcher on automatic, or that it was lunch time, so..... etc.
As to WHO the negligent party was, it's plausibly the separtists, it's plausibly some Russian advisor who was detached to help them, it's plausibly anyone who had one of these launchers. It's the kind of stupid thing people do when they're confused and harried. It probably wasn't the Russian troops. This isn't the kind of thing people do when they're part of a standing chain of command.