This could be anything from "you can't take a sick day" to "you have to answer my emails within an hour", it's a boss thing, he just had a controlling relationship, basically what we call "mobbing" in the work place. I wouldn't be shocked to learn Horner did mobbing, that is normal in the business world unfortunately and people who are that much at the top think that they are gods, that can't be questioned, and this could be the main reason why all of this started.
I hope there is no such scandalous thing happened with Christian Horner. Because it just doesn't make sense to me at all. He hasn't been such type of person so far. Why would he do such thing now to risk his job?
Mobbing is everywhere unfortunately if it is just the type of mobbing we are used to see in every work of field. If it isn't a more serious problem than that I wouldn't expect Horner to quit his job or to get sacked. Red Bull wouldn't like to have crisis situation while the new season is not too far away from us now.