Her tutors may be busy, they may have other teaching, they may have a further checking procedure, the senior person for the meeting may have a very full diary all sorts of things. I know my Dean, for example, who can sometimes be involved in cheating cases, is often away for 10 days at a time visiting partner universities in India, or wherever. My diary is pretty full for the next 2 weeks if this came up for me in the essays I'm marking all this weekend (I started at 6am this morning), I'd need to schedule it for about 10 days' hence.
So, there could be many reasons.
Frankly, it's not your call. And I'm not sure that you being away at the time the appointment is scheduled is something the university is likely to take note of.
Your daughter is seeing her tutor, and that tutorial should be helpful. What you can do to help is to advise her to seek guidance on referencing, and not to panic about it I sometimes think students' anxieties about the essay itself get projected onto referencing, as if that's the hardest thing about the essay whereas actually, it really is the easiest thing!
Academics are professionals. We know what we're doing, and we can be trusted to deal with this sort of thing without being unfair. We have procedures, and your daughter will not have not been told about plagiarism.
I'm afraid your daughter has contravened one of the most important professional principles in a university: not acknowledging the use of someone else's ideas and/or words. Even if it is not malicious or malingering or deliberate, I'm afraid she has to take the consequences. Most students in most universities have to sign a declaration that their work is not plagiarised.
Let's hope it's a rap over the knuckles. It sounds as though she'll take that seriously, and be much more aware of appropriate professional behaviour in future. Focus on helping her to learn from this experience, as difficult as it is for her.