I've seen it most often with those with ADHD or on the higher-functioning end of the Autistic spectrum (and I think it's most common in those with Tourette's) who can sit on the challenging behaviour for a limited amount of time - like a whole school day, or a nice afternoon with the grandparents - then go home and trash their bedroom, get aggressive towards siblings/parents, stuff like that. The child knows that the parent loves them so has no restriction on the behaviour they display because they're totally secure in that bond.
Plus, we as Mums have no end point to our time with our children. My mum looking after R knows that she only has to get through till X time and I'll be home to take over - I was the same at work, could take any amount of challenging behaviour because I knew that in a few hours I'd be in the pub with mates and a glass of wine, or sat watching TV cuddling the cat. It's a damn sight easier to be relaxed and just roll with whatever is happening when you know it's only for a short while. We don't have that, we're on call for our dc 24/7/365/18 or longer so the internal tension can still mount, even when the dc are down for a nap or the night - there's still that little bit of awareness that we might be needed.
I was dreading my trip to Jersey, leaving R for 3 whole days. But actually it's done me the power of good as I had no idea I was actually getting tense from lack of a break until I stepped away iyswim. And he had the most incredible time, it's been really good for him to be the youngest of 6 instead of the oldest of 1 for a few days. His language has come on unbelievably - today he said Money when I went to the hole in the wall, said "Door Meow" when we went past the door because I'd forgotten to go to the cash point and had to go back (he meant that Mille the Cat was the other side) identifies Noddy, has said a few people's names and I think we'll have 2 word sentences in the next couple of days. And the teenagers have taught him to fistbump!
Oh and we were at a festival this afternoon. I asked if he wanted lunch. I asked what he wanted to eat and he dragged me over to the burger stall. I read out the two options (it was a posh burger stall) and asked if he wanted Lamb or Sweet Chilli burger and he pointed to the Sweet Chilli words. Do you want onions? And he bloody ate it too we shared and I was hungry, half a posh burger is not an adult lulnch! ketchup, chilli and all
Then "Do you fancy another boogie, R?"