He has been very private about it. Not sure we will ever know
Good. People don't need to know everything.
I think Rubiales, Robert Moreno and the players know what's going on but no one in media has reported about that (at least I haven't seen anything). I doubt journalists in todays world would have enough moral ethic to respect peoples privacy if they would know anything so I don't think anyone of them really knows.
So people (mostly on twatter) have started a lot of different rumours. First I've read his dad was sick, then some people were saying Lucho has blood cancer and now there were speculations about his daughter being very sick/in critical condition. And I think this is the most likely reason for Lucho's familly wanting privacy and protect their under-age kid.
Whatever it is I wish Lucho and his familly all the best.