I can explain that madam by simply saying "I don't know". There's something obviously awol somewhere.
I was enjoying this problem of the 'Restarting' just staying there with that impressive little circle going around in squares and that went on for days.
Then suddenly & without warning, it just changed and I've not seen the 'Starting' hanging since. I have to sum it up as both weird and wonderful.
Lately, it was the computers not going into Hibernate or Sleep and suddenly it just stopped and worked ok. I have no answer as to why that is! Apple must be eating their little hearts out!