I don’t know the technical programming which has gone into it, but it was something to do with the key bindings not being created when joining a match. This could happen when the clients quit during a state in which the keys are not being used, e.g in spectator or quitting at a specific time. He added extra logging to get more info dumped into the logs and it gave him a bit more to work on.
However, it isn’t as easy as that to reproduce, so in theory there could be something else playing a part in it. That’s why the fix our programmer put in can only really come to light on live, as it could take days to reproduce, one positive is we haven’t seen it internally since he put his fix in so fingers crossed!