Re-read 9 July 2020
Ah, so the two parter starts with Hilda lying to her mother about her whereabouts. Because her adventuring had been curtailed, she had been increasingly lying about her mother by saying that she was either out with David or Frida, both of whom are from the Sparrows. (I wished that Luke Pearson had expanded more on their friendship and adventures, the same way that the TV series had. Can't have everything, I guess :) )
Hilda breaking more and more of her mum's rules led to her getting grounded. In an attempt to see the fireworks for the bird parade, Hilda and her mother get pulled through the cracks and were dropped into the middle of a troll cave.
On re-reading, it makes a lot of sense why the troll "mother" felt that Hilda was more troll than human: she craved the feeling of being free, adventuring, and running around with no limits. But as a mother, I understood Hilda's mum's perspective too: it's ridiculously scary not knowing where your child is at any one time, and when your child is free-spirited, and doesn't keep her word, any wonder Hilda's mother had been attempting to erect boundaries that neither were accustomed to.