Hormones apart, I think, Y5 and Y6 are stressful, DCs aren't grown up enough for real freedom, but they are leaving toys and childish things behind.
Bright 9 year olds find school very frustrating if lessons aren't well differentiated. I behaved appallingly in Y5. Slamming my desk, refusing to do work I thought was pointless and being cheeky. (I was better at home because my DDad was loving, but strict and I wasn't fed up at home).
By10 DCs want more control over their lives than they can have or actually handle. Most don't go to a laid back village primary with a class of 40 as I did. So it's home not school who gets the brunt of the bad behaviour.
Sometimes their frustration comes out in clear toddler grade stroppyness, sometimes in teen attitude and sometimes just in low level grumpy moaning.
They focus their frustrations not on big things, they know they can't control those, but stupid little things they feel they can. Trouble is their choice of things to get totally annoyed about is, to everyone else, not always logical and very wearing.
The best response I found with DD2 was to ignore what could be ignored and send her to her room when it simply couldn't be.
Then to give her the opportunity to feel grown up. Oh and lots of hugs.
Independence and being able to do things with their friends and a say in family days out etc.
With DD2 really little things like KFC rather than Macdonalds. A hour in town with a friend on their own or going swimming, helped.
As does being allowed to stay home while I taxi her sister about or getting to bake.
Hang in there OP it does get better, at 11, DD2 apologised for one particularly ridiculous tantrum and she's got to 13 without throwing another.
Yes she grumps a bit, she is a control freak by nature, being in control makes her feel safe and loved, but she's much much better.